Try Apple.com or failing that, eBay.com or CraigsList.com
The iPhone is a more or less brand new product; you aren't going to find it at most wholesalers and prices on a 20 count order are going to be atrocious.
One thing into which I've been looking is buying the iPhones and cracking the SIMs on them, then exporting them. There are a few tutorials out there and I think that demand for an unlocked iPhone would actually be pretty high (it isn't yet available in most markets outside of the US.) This would be especially doable if the iPhone platform could be hacked to trick the phone out (games, high security, GPS or whatever) but I'm not sure what the capabilities are at this point.
There's also a homebrew Linux based iPhone alternative being developed, Link hidden: Login to view This has a lot of potential for taking on the iPhone users who are in it for the eye candy.