Before I got CS:S, I used this site for textures. http://kajar9.wix.com/cscheater2#!downloads/chgd Under counter-strike: source, use the Mega Game Content one (not the maps). It is the fastest download there. After it's done downloading, extract it to your gmod addons folder. (Example: .\garrysmod\garrysmod\addons\*CSS folder* ) A simple search on YouTube will do too. youtu.be/yaL7gISscEY
Roger for the css textures i found out this site http://gmodtextures.com/ You'll get all the texues for gmod(CSS ,DDS ,PORTAL ,ETC.) try it