Is there any place to get Call of Duty Black Ops 1 cheaply?

Hey,

I wanted to go back and play zombies on Black Ops 1 with my friends, for PC. But the problem is, with CoD games the prices never actually go down. Call of Duty Black Ops 1, a 7 year old game, is still $40. That's so fucking stupid.

I want to play this old, dead game for cheap, but it seems like it's still 40 bucks everywhere. Anyone know where I can get it cheaply? I don't think it's ever going to stop being fourty bucks but I refuse to pay that much for it.