That's a wicked site and one you should add to the resources section if you haven't already. There's a thread in there about texture sources so do post.

I made a thread about making textures seamless here:
http://www.cartographersguild.com/sh...ut-of-anything

which has a free script using ImageMagick so you can automatically try it on all of yours and see which ones worked out ok. RobA converted the guts of the script into a GIMP plug in so there's one or two threads on that too - his sig has the links I believe.

I like the CGTextures site which is similar but we found that their license prevented the use of the textures in CC licensed works so I still use them but I can do a lot of what I want with those textures now.