The General Service Administration is the biggest national sales agency and you can check out fleet sale cars and trucks on their website. Online versions of the auction can be located through the GovSales website. Finding former police cars for sale is down to how often those local auctions are held, but you can also try eBay Motors. Government car auctions are there in the motors section for you to search through. You can search by ZIP code, type of car, miles from your destination or make and model of the car you require.
I wasn't aware that the TSA sold all of the stuff it confiscated! That's cheap goods right there! My mom works for a cruise line, and they don't confiscate stuff, but people do tend to forget all kinds of things they bring on the ship, so every once in a while the company has a big flea market available to employees. I find it funny when my mom comes home with watches and bracelets she wouldn't have otherwise bought.