Search all of Craigslist my friends...
http://www.searchcraigslist.org/
Plug in:
Poly 318 4bbl
Poly 318 4 barrel
Poly 277, 303, 318
any number of descriptions, and see what comes up.
I did this periodically (for months) while I was restoring my car. Some are listed quite high while others are much more reasonable. Found one but the guy wouldn't send photos and didn't want to ship. Found a 2nd one that had photos AND he was more than happy to ship. They're out there...you just have to grit your teeth and stay at it.
Oh, and neither were half of what that ebay item brought.