Latest version of iOS that will run on an iPhone 3g is iOS 4.2.1 per
If the app you are interested in needs a later version of iOS, you
will have to purchase a newer iPhone. Decision as to what versions
of iOS are supported are made by the developer of the app and not
To find apps for older versions of iOS (quote from someone else's
post but unfortunately I lost their user name - apologies to them)
iOSSearch - search the iTunes store for compatible apps.
Vintapps 3.1.3 - paid app
Apple Club - filter apps by iOS version.