The Tech Report did a little digging and concludes the availability of AMD's Radeon HD 7770 and 7750 doesn't appear to be that great:
Based on our findings, Newegg seems to have gotten the bulk of initial shipments. Amazon has not one card in stock. An unlabeled MSI Radeon HD 7770 is available through the Amazon Marketplace, but at $200.64, it's prohibitively expensive. Meanwhile, TigerDirect and NCIX each have all of three cards ready to go.
The Radeon HD 7750's availability situation is even more dire. Listings are hard to come by, and we only found a single model in stock. At least it wasn't marked up above AMD's suggested e-tail price.