It can be a problem to find a used engine if you are
in a hurry. Seems like most of the time they just all of a sudden "appear", usually in the form of a cheap car with other issues.
I had an extra 240 engine, and somebody was going to buy it. They decided to rebuild their own engine, and a week or so later here is a cheap little 240 on Craig's List with a seized engine.
So, what I'm trying to say is that usually "all of a sudden" they just "show up". It is a marvel of nature.........
Happy hunting!