There is a website for Bulk Apothecary. I think that is the name. I don't know if the castor they sell is suitable for our purposes. More investigation is needed.
They have castor in various container sizes. One size is 8 lb., which appears to be a gallon, listed for about $17, plus shipping, last time I looked. This was months ago.
If we can use this castor then it is the best price I have seen anywhere. Otherwise, I get castor from Sig.