I agree that finding the fabric is the hard part, but once you have made one, you will have enough leftover fabric to make a dozen more. I was always glad when the shops were willing to cut 1/8 yd pieces for me!
You really need a vertical design wall to make a colorwash quilt. I did it the cheap-and-easy way. I bought a $2 vinyl tablecloth at walmart and used a Sharpie marker and my 24" long acrylic ruler to draw a 2" grid on the fuzzy back side of it. Then I cut away the excess tablecloth and thumbtacked my "design wall" to the wall. You want to put it in a location that is convenient to stand close to but also where you can move back and look at it from a distance.
Here are some of mine - http://www.gloryquilts.com/colorwash.htm They are a few years old, and the photography is poor; I am better now! ;)
And this is a very short article/list I have on my website about selecting fabrics for colorwash quilts - http://www.gloryquilts.com/colorwash_quilts.htm
I hope it's ok to post those - I'm not selling anything there!
Hope that helps!