Do you know any websites to buy games no longer available on Steam?

If you want them cheap go for the Console Sale bin.
I doubt anyone is going to sell them for Cheap on the Steam Version at this point.
There is the Arrrr way but thats only up to you if you really desperate to play the games.

Collecting old and rare stuff will always be expensive and the longer you wait the scarcer the supply will get.

people trading those on steamtrades barter,vg will tell you, it is rare, hard expensive etc
but you can find SOME of those via keyshops or some legit, but ie russian steam key distributors (or sometimes, russian not official shops lol) way cheaper that from traders

If there's any seller known for removed games on steam, Valve would want to terminate the relationship with them XD

Found James Bond game on cd key site. But it's RU/CIS. Anyone would like to try if it's the same case as with The Amazing Spider-Man RU keys? :)