It's difficult to figure out when impounded scooters will go on sale. The approach I've used is to follow stories of cities picking up Birds and keep an eye on their police and towing auctions. I'll keep this thread updated with my findings and make you aware of any scooters at auction I come across.
If scooters go up for auction they'll be listed on this website - Seibert's Auto Auction. Auctions occur the second Monday of each month.
Scooters Impounded 12/05/18
Athens-Clarke Commission grounds Bird scooters
West Lafayette, IN
Scooters Impounded 12/11/18
News Article: Bird scooters - West Lafayette resorts to towing as promises fall flat near Purdue
Once you purchase a scooter you'll need the hardware to hack it.