https://archive.org/details/softwarelibrary_palm is a good place to start (and you can just run them in-browser via cloudpilot.)

Back in the day I think most people got their apps from the PalmGear website (https://web.archive.org/web/20010331040533/http://palmgear.c...) and sync it to their Palm device via HotSync (later Palm Desktop)

PalmDB