Hi. You can get some good deals on refurbished iPhone 4 models from Amazon or Ebay. Some people sell temporary unlocked iPhone models for more cost. Such iPhone models would get re locked on future iOS updates and it has to be unlocked again resulting in more expense. Instead you can get a network locked iPhone at low cost and unlock it yourself safer permanently.
To unlock an iPhone 4S visit http://www.superunlockcodes.com/apple-iphone-4s/rs16wp9/ and get the remote unlocking service. This method of unlocking is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too.
Considering that the iPhone 4 was released in June of this year there probably aren't many used or refurbished ones yet. Finding used 3GS's should be no problem if you don't mind being one model behind.
Any AT&T store
they usually run 150 for a 16 gig
or 250 for a 32 gig
i recommend the 32 gig
You can definetly get the i phone 4 at att&t. My brother got one there and it is awsome!love ya