I do all my Textures on a program called Gimp. although I'm not into the isometric stuff just yeat I have found a plugin that might help with turning normal textures into isometric ones.

http://registry.gimp.org/taxonomy/term/1224

hope the link helps.

As for tutorials I suggest looking up YouTube videos as that's where I learned many of my skills.
here's a video that may help. https://www.youtube.com/watch?v=cdxJQFXsxXQ