Where to Get iOS 4.2.1 Custom IPSW for iPhone 3GS?

Hello Armand, there are numerous website and blogs offering custom firmware for your device. However, we highly recommend to our readers to create their own custom firmware as there are possibilities that these firmware can be damaged or corrupted especially if you have download them from some unreliable source. Additionally, this process is not ideal for users who have poor Internet connection. Anyway, you can use Sn0wbreeze (if you are on Windows) or PwnageTool (if you are on Mac OS X) to create iOS 4.2.1 custom IPSW.

If you are using a Windows, then you can follow this tutorial on how to jailbreak and create custom iOS 4.2.1 firmware using Sn0wbreeze 2.4 beta 1. You can get the stock iOS 4.2.1 from this link. On the other hand, Mac OS X users can refer on this blog post on how to use PwnageTool 4.2 to create custom firmware. Once you have successfully created the pre-jailbroken and custom iOS 4.2.1 for your device, you can now restore it into your device using TinyUmbrella. The steps on how to downgrade via TinyUmbrella can be found here. Also, you should use iREB when restoring the custom firmware in order to bypass all 16xx errors, use this guide if you don’t know how to use iREB. Lastly, you can unlock your iPhone using Ultrasn0w 1.2.3.