Where can I buy a aerator for my barrel to make liquid fertilizer?
If you're looking for a mechanical/automatic aerator that will operate on it's own, you'll probably have to build something. If you just want to be able to whip it up and stir things well then I recommend a mortar mixing wand and a half inch drill. These wands are about three feet long, so if you need to get to the bottom of the barrel, you'll have to buy or make an extension for it.
Or there is something like this:
I believe when they did it on Ask This Old House they used one for a fish tank. Check with a pet store and I would say get the biggest one possible. Also check with manufactures of Bio-Diesel equipment (that is use to home brew Bio-Diesel) I believe that they aerate the fuel with one used for a fish tank but they may have something more heavy duty.