If you just want to create textures for existing clothing items, all you need are UVMapper Classic and a paint program like Photoshop, Paint Shop Pro, or The GIMP. You might also look at scourge's MAT Pose Edit in Free Stuff if you'd like to have MAT files to automatically apply the textures.

Renderosity has a Texturing forum you might find helpful, along with forums for Photoshop, Paint Shop Pro, and other 2D paint programs. RunTime DNA has a texturing forum, also, although it doesn't seem very lively at the moment. And PoserPros has both a clothing forum and a texturing forum.