To more easily find compatible apps:
iOSSearch - search the iTunes store for compatible apps.
Apple Club - filter apps by iOS version.
Starting when iOS 7 was released, Apple now allows downloading the last compatible version of some apps (iOS 4.2.1 and later only)
You first have to download the non-compatible version on your computer. Then when you try to purchase the version on your iPod you will be offered a compatible version if one exists.