There you go:

http://www.thegamecreators.com/?m=view_product&id=2056

They have a lot of good models but you need to convert them from .x to .mdl. I've written a tutorial here (in english, just click the download link on the bottom):

http://www.jofre.de/?p=693

Terrain tutorial using Shade-C
amBx-Plugin - use amBx in GameStudio
3D Tower Defence