They pop up from time to time on e-bay (in fact there was a rebuilt B20 head up as late as last night). It isn't that hard to find them kicking around junk yards with lots of miles (there is a '75 245 with a B20 in one of the local scrap yards here - 175K miles). It seems that some parts of Canada/USA have one every few feet (mostly west) while the east has none. You may have to pay for shipping, but don't pay more than $200 for a rebuildable core long block.

It took me a while to find the B20 that I'm working on, but they are inexpensive to have rebuilt. So unless you are pressed for time, I'd look for a rebuildable core and do the rebuild locally or yourself.

Craig