The pretty recent post of a celebrity (can;t remember his name off hand) buying AR's for the Phoenix PD, was one way.
Other PD's utilize seized property sold at auction, cash used in drug arrests, etc,.. for additional funds. FOP, PBA, etc,..
FWIW, Massad Ayoob wrote an analysis of the N. Hollywood shooting, and summed it up with training. One decent shooting officer, with a 30-30, could have ended the situation, in 42 minutes less than the whole event took place (44 min.)