I use medical grade castor from a Walgreens pharmacy. You can usually get it at any good pharmacy, and its not a whole lot pricier than bulk hobby shop stuff. If you are only mixing a small batch, it can be a good way to go.

(Prior to SMALL this year, I promised to provide fuel for Skaliwag, who was coming out of town. I got the fuel, but I forgot to buy a large bottle of castor at the LHS before it closed. I stopped by Walgreens on the way to the event and picked up a bottle. Was enough for all of our .010s for the whole weekend....)