Use Kelley Blue Book or Edmunds to find the fair purchase price for the specific model and mileage.
Some off-lease inventory is sold at auctions like ACV Auctions (for licensed dealers) or Exleasingcar (international). How to Buy an Off-Lease Car (Step-by-Step) where to buy used leased cars
If you want to take over an existing lease or buy it out mid-term, platforms like Swapalease and LeaseTrader connect you directly with current lessees. Use Kelley Blue Book or Edmunds to find