If you want a poly/cotton like RTW scrubs, there are several fabric websites that carry them. Fabric.com usually has a few colors to choose from. Just google medical scrubs fabric. The fabric is a little heavier than the poly/cotton broadcloth you find at your local fabric store.
Personally, if I was making my own scrubs, I would want 100% cotton, and quilting cottons are very close to the same weight.