I went to Tractor Supply Co. in Lucedale looking for my tubs. I had never been in the store before. What a wonderful place! Stuff that I didn't know where to find, they have it. It is on my VBS Mad Science shopping-go-to list from now on.
I got a 16 3/4 gallon tub, a 7 gallon tub, and a 2 or 3 gallon bucket (I can't remember which, but it doesn't really matter.) I got a 2 cubic yard bag of potting soil at Wal-Mart to fill them up. I was a little worried that it wouldn't be enough dirt, so I put an empty gallon milk jug (lid on) in the bottom tub in the spot where the smaller tub would rest over it. No need for lots of soil depth under the next tub. (be sure to rinse the jug well first. souring milk releases gases that can blow the lid off and I'm not sure what that would do to the tub above it.)
I planted oregano seeds in the top tier, due mainly to a vague remembrance that oregano can take over a container or even a garden.
I am looking forward to the day I can smell all those herbs every time I walk out the front door.