I bought as much cotton as I could because I am using my own DIY detergent and cotton would be the less risky fabric to use. Easier to strip to if it just doesn't work. Note: so far my DIY detergent does NOT impact any of my cotton prefolds or inserts for my fuzzibunz, trend labs, and random chinese diaper that I'm not sure of what it's called, lol.
If you're handy with a sewing machine you can purchase a yard of bamboo fabric to make your own inserts to play around with instead of paying a bunch of already made