Maybe its just the botanist in me and all but...
"good source of cheap plants"?????? Please don't clutter that MASSIVE gorgous thing with anything that us normal sized viv guys have! You have the space and the setup for some amaizing things that we normally can't have. Spend the little bit extra and search around and get stuff that will do that unique viv justice. Please don't go to Lowes/HD for your plants! We all do for our little ones but you've got the potential, space, and volume to fill, so order from specialty nurseries. I mean, you didn't fill your coral setup with just zooanthids right? Don't let us down! You can do it! If nothing else at least get a brom assortment from some of the guys on this forum. I have and have been quite pleased with the variety/colors I got.
Because I'm a plant nerd I go nusery hunting around south texas from time to time looking for cool plants and places to get them. Look up all the places around you and go visit them. If you plan it out you can get about 4 in a day depending on distance. The more obscure/ little old lady run the better. I've found all kinds of weird stuff that way. Online I would hit up:
aqua forest aquarium
For your aquatic plants. All kinds of cool aquatic brophytes to choose from.
Bird Rock Tropicals Home Page
This place is run by my bosses extended family. I can vouch for their quality. Call them too, they might not have everything listed.
Tropiflora's Cargo Report
And of course tropiflora. These guys are really good about putting up pics of what they have in stock but call them too. If you tell them your looking for some odd/unique tropical stuff they might have a few things unlisted.
And then.... if your REALLY into it. You go visit Hawaii, Costa Rica, Cozumel, tropical paradise of your choice, and bring back, minus soil, cuttings of your own. I've got broms from Costa Rica, moss from Hawaii, and a few ferns from Cozumel. Yes you do have to go through customs with them for Costa Rica/Cozumel, but as long as it doesn't have soil and your nice, they don't give a dang as long as its not pot.