Car Auctions Web

Lonestarauctioneers.com (Texas), maybe more where you live.

And of course EBAY.
References :
http://www.lonestarauctioneers.com/

Police cars are generally sold at government auctions. The auctions can be federal, state, county, or city. Such auctions are held around the country. The vehicles being auctioned are usually high-mileage but well maintained, and are being replaced with newer models.

The following article describes the auction process in some more detail:

http://www.leaseguide.com/Articles/carauctions.htm
References :

Not to be a wet blanket, but these cars haven’t been used, they’ve been used and abused. There’s a reason that these cars are auctioned off at 60,000 miles as they’re driven much harder than the average family sedan. The battle scars on many police cruisers bear witness to that fact, Just watching the police chase shows would discourage me from buying one.
References :