What JED said.
Once you become aware of them, you see used tanks all the time.
It is not at all unusual to find good-sized tanks for free, or under $100.
People just want to get them out of their basements or garages, or don't want to move them.
You just have to be willing to wait until the right deal comes along.
New tanks themselves are not all that expensive.
And those are all kinda off-sized tanks.
If I'm paying more than a token amount for a tank, I'm going to buy new.