The most common flats I've seen are birdseye ones. I got some from www.cottontailbaby.com. A lot of stores have them though. There are also flats made from velour or hemp but those can't be snappi'd (but would be more absorbent). I think regardless of material they are easy to wash and dry because they are so thin.
Try a search now and I'm sure you'll come up with some more sites.
I ditto the birdseye - you can also get them from www.diaperware.com. Another option that I like are the *flour sack towels* that you can find at Target or Walmart. They are 30"x30", can snappi easily, and also dye up really nicely if you want to do that.
Thanks for the suggestion, I will check those birdseyes one out. Snappi shouldn't be an issue since I am going be be using those in covers (most prob.)
Does anyone had experience with hemp-fleece flats? I have some hemp-fleece lying around here and I am thinking stitching 2 layers together to make a flat diaper. What should be the size for a 20 Lb, 14 month old child (kind of skinny)?