Amazing thread, and knowledge here.
I have a source for instrument grade propane (R290) at 99.2% purity.
It is Scott medical gases here is the link
http://www.scottecatalog.com/Scottgas.nsf/web/MedHome
You will have to call for a quote. I purchased a case (12X 400gram cylinders) for $170
This is what we use in the cascades I work on.

Mytec: please check your web site email

Thanks.