Diablo – the spawning areas in the Southern Highlands

Towards the beginning of Act III, you will find Fallen and some equivalent mages who can generate more Fallen.

If you avoid killing the mages, you can have fights that go on endlessly. It took me five minutes to fight my way down some stairs once because the Fallen would never, ever stop coming.

After several days worth of research, using the items offered at the Auction House as a guideline, I was able to determine that socketed items are generally available as low as level 15. It does not seem to matter what item is in question (armor, rings, weapons, etc) - you must be level 15 to equip something that contains a socket.