Those places sell a couple kinds. One is in a smaller package and has fertilizer and perlite in it. Don't get that one! The other tends to be in a very large bag (50lbs?) and is kept outback in the mulch and stones section (at the Lowe's I went to). That is peat with no additives. That's the one to get. I paid 8 bucks for it. Hope you have other uses for it, lol.
Then there is sphagnum moss in small brick shaped bags, kept in the plant section, indoors if I recall. Might as well get some of that too. There may be a couple different kinds. One is green and filled with twigs and stuff and gets ooky when wet. The other is straw colored and much cleaner. Get that one. About 5 bucks I think.
I've been buying Mosser Lee for years and I'm not overly happy with it. It does have a lot of sticks and straw and just extra "junk" in there like you say. When you fluff it out it looks dirty (as I understand long fiber sphagnum, this is from the darker lower layers of sphagnum rather than the whiter top layers). Dang cheap though. Compare that to a $12 brick at petsmart of the better stuff. It's a smaller brick too if I remember right.
By the way, does anyone have a good source for cheaper higher quality sphagnum? I've tried some craft stores, but I can't seem to ever find it locally (other than pet-smart/-co)
I use Better-Grow Premium Grade Orchid Moss. It has American Orchid Society on the bag. It comes in a brick like bag, similar to pet stores except larger, is good quality. Between 4-5 bucks. I find it at Lowe's and Home Depot in the plant section, near the Mosser Lee.