AnkiDroid for Android works well, I used it for a while when I didn't have an iPad (old one had died, had an Android tablet collecting dust so switched to it).

AnkiDroid for Android works well, I used it for a while when I didn't have an iPad (old one had died, had an Android tablet collecting dust so switched to it). bbno4 already pointed you to the shared decks. Some people do sell decks. The main issue with that business model is it's entirely trust-based. The decks have no form of DRM or anything, they're just sets of cards and media that get imported so can be freely passed around once purchased. But some people do sell them.