This site[0] is going a little out of date but has good info for several-generations old Thinkpads right now. As a sibling mentioned, Ebay is your friend. The other nice part about Thinkpads is that they are quite user serviceable and you can usually buy any part you need for reasonable prices.
[0] https://www.bobble.tech/free-stuff/used-thinkpad-buyers-guid...