Apps for an old iPhone

I'd like to seek and find apps that will work on iOS 3.1.3 – without wasting time browsing apps that will not. And without a jailbreak.

Apple's App Store appears to lack this capability – no filtering out of incompatible apps, even when using the Store on the handset.

I found one answer … additional answers will be greatly appreciated.

Thanks

Background

VintApps 3.1.3 might have been ideal. I never tried it.

There's an iOS category at MacUpdate but the filters are nothing like what's required.

Note

I'll use my first generation iPhone for as long as it continues to work as well as it does. So please, no answers that suggest an alternative iPhone.