We used/use the gerber trainign underwear. You can get it at babies R us..or pretty much anywhere that sells gerber clothes. If you want regular underwear GAP makes very small and Target sells small sizes.
We use Gerbers too and just ordered Simply Pullon covers from the EC store. The Gerbers are not very absorbant so if you need something more absorbant, I suggest throwing in a liner or prefold for more protection.