I bought some from Dennis at Hot Metal Engineering a while back - they work pretty good in my Nibbler. Still planning to build a dedicated machine with some throat depth.
They were on sale - no idea if he still does them
http://allmetalshaping.com/member.php?u=5734
I also have two homemade sets (one very small like in Ben's "Shrinky" machine) and they work equally well after some tuning....