I have a lot of different types of terrain.
For cardstock, I second the recommendations for WorldWorksGames and Fat Dragon for Fantasy. EbblesMiniatures is great for SciFi, as well. There is also another company, Paper Miniatures AT, that does some very cool desert buildings and an Asian temple (I think you can find him on RPGNow). I really love me some cardstock terrain, and I have bins and bins and bins and bins of it - all that I've made, and I'm about the least crafty guy on the planet.
When buying premade terrain from ebay, which I also do, especially for irregular shaped items that its hard to make with paper (like good looking rocks), I typically search for the term war* terrain. That gets me the most results. There are a few really innovative sellers out there, but like always on ebay, check shipping, check seller feedback, check their location, etc. (There is currently a seller from Brazil selling a few nice items - but with 0 feedback, and $20 shipping.)
You might also check out Dundjinni or Campaign Cartographer. I use them to create battlemaps that I put under the terrain. A simple, but good looking battlemap can go a long way to making the rest of your terrain look even better.
Edit: Almost forget, but you can find some nice, if quirky, terrain at pet stores (try the big chains - PetSmart, etc). Its not painted the best, I've found some very nice looking terrain bits (rocks, ruins, bridges, hills, foliage) at decent prices.