www.cgtextures.com - REALLY cool site, and it's all free, though there is a limit to your daily downloads.

Blend it with the texture in Photoshop, or blend it into the final render in Photoshop (you'll need to do some adjusting for lighting conditions - highlights, shadows, richness and color, etc.). Either way, find some pictures of moss and have fun.

Hope this helps-
-Lew ;-)