Along with duct tape, foil tape, and duct-seal putty the five gallon bucket is one of the staples of hillbilly engineering. You pretty much can't have too many.

Don't forget that lots of building supplies come in five gallon buckets. Paints, sealants, drywall mud, wallpaper glue, concrete additives, etcetera, etcetera. Some of those you don't want in close contact with food but one of the benefits of the shift to low/no-VOC compounds is the general trend is that these materials are becoming far less toxic.

On a large job site a few years ago I was picking up twenty or more buckets a week. And there were plenty more.

Large institutions may have shifted toward large drums, typically 40 or 55 gallon drums, another fine resource, but many have stuck with five gallon units for their cleaning and floor maintenance supplies because they are easier to handle and dispense.

Nice design on the self-watering pot. The one addition I would make would be to line the bottom half of the upper pail with landscape fabric to limit the growth of roots into the lower compartment.