Thanks a great idea. I don't have a site to recommend, but you can always do a search for free texture maps. For some of the texture maps, it will list that it is legal to use and distribute, so it may be more of a case by case basis for distribution purposes.
If you are not distributing the image itself and distributing the rendered curtain instead, then it may be a lot easier legally to do this. Hope this helps. We'd love to see what you come up with in the end!