Probably a good place to start is RobA's tutorial on creating an artistic map in GIMP - its probably the most read tutorial thread on the board. Note the search box at the top right of this site - just enter "GIMP tutorials" and you'll find a bunch.

I noticed your thread on ENWorld boards, and was going to point you here, but seen someone already did in that thread. Good luck in your search for a cartographer, but I'd recommend, if you are somewhat familiar with GIMP to follow the tutorials, make your own map and save you some money! I do commissions too, but you can't afford me, and I'm too busy anyway. Good luck in your endeavors.