I don't know what products are available in Bulgaria, but if you don't mind ordering from abroad I hear good things about Epotex aquarium epoxy.
If you only need small amounts I don't think shipping will be that much.

The website is in Swedish but you can probably work it out with google translate, their e-mail can be found at the bottom of the page.
Epotex Akvarie och terrarie

 

ok I apologize in advance, but I'm going to go on another polymer clay rant. I am a professional polymer clay artist and let me tell you, you have to be really careful what you put on polymer clay. It has a bad reaction with nearly everything. sometimes even months or years later. I have heard horror stories from other artists with clients coming back to them a year later pissed off because their $300 sculpt is now cracking or sticky. most people use either liquid sculpy or fimo as a sealer or the sealers made from polymer clay by its manufacturer or its generally accepted that duncans matte sealer will hold up over time and is what I use. however those are all toxic.

Polymer clay itself is in fact toxic. It will leach toxic chemicals over time. its not food safe. It will also break down in wet/humid conditions over time. Polymer clay is actually pretty toxic stuff. you arent even supposed to bake it in the same oven you bake food in. I have a dedicated oven just for my clay work. I dont know why Fimo claims its aquarium safe, because it is absolutely not!

I would never ever put polymer clay in a viv or aquarium. I dont care how many people have done it or what they sealed it with. Its just not really a good idea. I suppose you could try completely encapsulating it in resin. I have never tried that so I don't know for sure what would happen. the raw clay will actually MELT resins and plastics (I have tried that), but properly cured clay should not.

there are plenty of epoxy clays that are safe and dont need to be sealed and some can even be colored directly and dont need to be painted.

or you can do what I am in the process of doing.....sculpt your mushrooms and make a mold and cast them in resin or silicone. I have a whole bunch of various types of mushrooms and fungi sculpted and ready to be molded. I just haven't had time to mold them yet. The nice part about making a mold is I can make as many as I want without have to do a whole bunch of sculpting.

 

Wow! I am really happy that someone who works with polymer clay professionally replied. I knew it had some level toxicity but i thought that when it cures fully there wouldnt be any problems. My mushrooms aren't expensive and are small, about 2 inches tall, so i am more concerned about the aquatic life in the aquarium itself. Not entirely sure how i would seal the mushrooms in epoxy. I have a polyester resin so I could try that. If you have experience with it feel free to share.
The only Epoxy clay I found is "milliput". Not sure if its a good brand but i might as well give it a shot. If i do with what should i paint with? acrylics before they dry? I also have some oxide paint for cement which is a powder.
Whats your opinion on polymer clay in a normal terrarium. There isn't any heavy water to break it down i guess.
I have been thinking about making the mushrooms with a couple of molds multiple times but then they will become all the same, although they really aren't that much different. And i am not entirely sure with what to paint the polyester resin. Again i have the oxide paint, which in the description says its suitable for resins but some say it wouldn't work for polyester. I know some powder or liquids colors are used in the batch itself. Another way i have seen is applying some of the colorant on the mold at specific areas so it grabs a pattern or a patch.
There is a youtube channel which gives really nice information molds and casting, not sure if i am allowed to link it.

 

polymer would be ok in a relatively dry terrarium, but anything high humidity is not a good idea, like I tell people who buy my sculpts, not to put them in bathrooms, because the humidity from the bath/shower can damage them. or dont put them in bright light or direct sunlight or let them get too hot. when even the cured clay gets warm it becomes soft (and can release toxic fumes) and brittle and will break. now too hot is 90+F so not really relevant here since that would be a bad idea in a terrarium.

I really hate sculpting with epoxy putty clays, but the one I use is apoxie sculpt. it comes in colors and I have also had really good success coloring it with mica powders mixed into it and rubbed on the surface before its cured. some epoxy clay is toxic as well. I did a search on the forums here and did a bunch or research before I decided Apoxie sculpt was probably safe. I have a rock wall I made from it in my auratus tank for over a year now with no problems with any plants or frogs or their froglets. there is also zoopoxy which there is a thread about here somewhere and Smooth On also has an aquarium safe epoxy clay which I will be using in a viv I am working on now. Im now sure how available to you any of those would be.

with resins you can color the resin with a base color before you pour it into your mold and what I usually do is color the mold itself, I usually use mica powders for this too. if you really want to paint it you can seal it with a clear coat of resin. what you can also do it pour your resin in different stages and use different colors of resin each time.

most of my mushrooms are going to be made from silicone with some of them having resin stems. I don't mean aquarium sealant or caulking I mean the stuff used in prosthetics or animatronics.

 

Thanks again for the great reply. I contacted the manufacturer, Milliput and they said its used in tanks and gave me some nice links, it just needs to be washed when cured but nothing toxic. So its another brand to consider if needed

It seems i am going to leave the polymer clay objects for out of aquariums use only.

I am thinking of using ground dirt and rocks as another dendroboard fellow said he does, since i am almost sure i won't find mica that's not meant for nails.

If you don't mind me asking why prosthetics silicone. Wouldn't it just make it hard to bond resin and silicone together. And how is the silicone better? more flexible and durable ?

 

polymer clay is great for sculpting. I love it (obviously)! its easy to use and you can get great detail from it, which is why its so tempting to make cool stuff for vivs and aquariums. Maybe it would even be just fine, but why risk the health of your animals for a couple mushrooms when there are safe options!

ground rocks, minerals and dirt are all great additives for epoxy and also for resin. The resin I usually use can have a very high percent filler in it so I could make fairly lightweight rocks that look and feel real due to a high percent of rock dust in them. I havent done it yet but I probably will. you can check craft stores in the stamping/scrapbooking sections, they often have mica or mineral powders. At least around here they do. I often use pearl-ex powder.

the silicone I use is a 2 part mix that is great for highly detailed projects. you can get it in different hardness for different projects its also easy to color and east to paint and its safe and its totally cured and ready to go in a tank in less than an hour. it picks up the tiniest details (its sometimes used in forensics to cast fingerprints) and is very durable.