I buy it by the 5 gallon bucket on Amazon. If you have an Amazon Prime membership, you can get free shipping from many of the suppliers and with a little shopping around, find a good (or more reasonable) price as there are quite a few suppliers of it on Amazon. I have found that to be far less expensive than buying single gallons at a local auto supply store, etc.
A tip I learned from Marcus Anghel is to buy a reasonable quantity of it (several 5 gallon buckets at least) and store it all in a plastic garbage can...similar approach to the 55 gal drum but a little less spendy. That gives you a vessel large enough to dip some fairly large parts into as well as being able to do a lot of parts at once...which is extremely handy if you have a full blown restoration underway given that, as mentioned, a 24 hour dip is really best for complete removal of the corrosion.