My suggestion based on original fo1 fo2 idea.
Item can spawn in lockers by formula like this:
(x~1000shelves/lockers)/(y-some-random-number)x(x~players-online)+some algoritme which making item spawn in random town/shelve, for example like shuffle in winamp. i mean random spawn algoritme. + formula for some random time spawn - 15min-180min for example.