Turbosquid isn't usually a very good resource for me. Sometimes you can find real gems in there but the quality isn't consistent and you can never much of what you're looking for.

Dexsoft produces some excellent game-ready models but doesn't have a very large selection. If you're willing to go down a little lower in quality, Blattsalat is a forum member here who will be able to whip up some custom models for you for a relatively low cost.

By the way, I've seen some of the stuff you've made so far with Gamestudio, and I have to suggest that you invest some time and money into getting your hands on some higher quality textures. Standard.wad textures used to be decent 10 years ago, but nowadays they just look bad. At the very least, I suggest you go to the Download page here on the Gamestudio website and buy a copy of the HiRes Textures collection. It's only $20 and gives you the same textures from standard.wad in a much higher quality.

EDIT: Now that I think of it, the Download page also has several links to some 3rd party websites that offer lots of other artwork and resources. You should check it out.