In the United States the iPhone is only sold at AT&T and Apple Stores. Now Best Buy officially announced that they will carry the iPhone 3G and become the first independent reseller for the iPhone in the U.S. AppleInsider reported about this deal before it became official.
Best Buy will offer the iPhone 3G at 970 Best Buy stores in addition to 16 Best Buy Mobile specialty outlets beginning September 7th.

Last week, Best Buy announced it had completed a two-year conversion of its stores to include upgraded cell-phone departments under the Best Buy Mobile brand. It has upgraded its computer systems to handle cell-phone activation and spent 250,000 hours training its employees.

Score said Best Buy Mobile, which resells phone service under the carriers' brands, now has a 3.6 percent share of the cell-phone market, up from 2 percent last year. It is a joint venture with CarPhone Warehouse Group PLC, which sells the iPhone in Britain and is Europe's largest cell-phone retailer.

In Europe, independent chains like CarPhone have a much stronger presence, while the U.S. market is dominated by the carriers' own stores. AT&T, the exclusive U.S. carrier for the iPhone, sells it in more than 2,000 stores. Apple has 189 U.S. stores.

It looks like the brief iPhone 3G shortage a couple of weeks ago is over. I just checked the inventories on the Apple site and most stores have iPhone 3G devices in stock.
Via this Best Buy press-release.