I think the biggest selection is going to be at Apple and AT&T. I got the Speck CandyShell from the Apple store. I saw a selection of 2 or 3 at Best Buy today and I know my local Target doesn't have Speck cases for the iPhone (thought I saw some for the iPod touch though)
I ordered a brand new one from Speck and they offer free shipping. It arrived in two days. Only store in my local area that sells them at this point is Apple.