1. Contact the developer of the app for a specific version of that app for your device.
2. If you have purchased apps in the past using iTunes in your PC, then you may have backed-up .ipa files. Use them instead. (Make sure that they are bought from your own Apple ID).
Other than this, you have no choices buddy. The device is unsupported now.
iOS 4.2.1 is the last iOS supported on that device.
You will not be able to run newer iOS apps, but you still can choose from well over 60,000 vintage iOS apps.
The trick is finding them. One way you could try,
There are other easier ways also.