I found these on amazon. It's the best price I've ever seen for quick hitch bushings/adapters. Figured I'd share. $16.99, shipped (if you have Prime).
If you have problems hammering the roll pins in, you can squash the tapered end in a vice. I didn't bother, and it was a bit of a pain to get it in, but not too bad.
EDIT: Apparently, you can get these at Walmart for $7.20 a pair. Thanks, CADplans!