Celtic Cloths: I love their glossary of different diaper fabrics and how to use them. They carry PUL, Hemp, Bamboo, Suede Cloth, Organic Cotton, and notions such as FOE, Aplix, and size tags.
Natures Fabrics: They carry a very wide range of diaper making fabrics, but I particularly love all the different wool they have. They have the most extensive line I have seen.
Diaper Sewing Supplies: LOVE how extensive their PUL section is. I also love their wide variety of FOE colors, and Kam Snaps.
Babyville Boutique is the newest line from JoAnn Fabrics. There is a whole section in the store dedicated to diaper making. And they sell in small quantities, so if you are just starting out sewing your own diapers you can experiment and play around.
The Fabric Fairy: The have tons of cute prints on cotton knits, great for making fitted diapers. They even sell “diaper cuts."
Very Baby: Their most extensive selection is in the microfiber and terry section.
Wazoodle actually sells precut diaper making kits. This would be very helpful for a beginner. They also carry ProCare and Zorb fabrics, which are the latest “technological innovations” in diapering!