Where can I download iOS 6.x firmware files for the iPhone 3GS?
Apple decided to support the iPhone 3GS with iOS 6, although with the release of the iPhone 5 the hardware is now discontinued. Some features are limited on the older hardware, however iOS 6 is available for OTA or iTunes-based update for 3GS owners.
iPhone 3GS firmware version iOS 6.0.0 can be found here: iPhone2,1_6.0_10A403_Restore.ipsw
iPhone 3GS 6.0.1: iPhone2,1_6.0.1_10A523_Restore.ipsw
iPhone 3GS 6.1.0: iPhone2,1_6.1_10B141_Restore.ipsw
iPhone 3GS 6.1.2: iPhone2,1_6.1.2_10B146_Restore.ipsw
iPhone 3GS 6.1.3: iPhone2,1_6.1.3_10B329_Restore.ipsw
iPhone 3GS 6.1.6: iPhone2,1_6.1.6_10B500_Restore.ipsw