Two good resources to find retired state trooper vehicles for sale are CopCarsOnline and Gov Deals. CopCarsOnline only sells retired police vehicles. Gov Deals sells a wide variety of government surplus items, including police vehicles.
CopCarsOnline's facility can house over 20 police vehicles at a time and also has an on-site police car museum. It offers free airport pickup for people intending to buy from them and spends an average of $800 refurbishing each car it sells. CopCarsOnline is located in Largo, Florida.
Gov Deals, which offers police vehicles from across the United States, is an auction style site that has potential buyers bid on the desired item. Shipping is not available for most items, and bids can not be retracted.