I have read this question regarding App Store updates for older iPads, and I'm in a similar situation to MrDaniel regarding my hardware / software.
This is a different question though: how to find apps, via App Store or otherwise, which will have a version which is compatible?
In the App Store you can filter by iPhone or iPad, but not by iOS version.
If an app has a compatible version, you can buy it through iTunes on another device, and then on the iPad you can download a compatible version via updates. But how to tell which apps you can do this with?