Geez! That size was a rare find for me. Just stop when you see a landscaper and ask. We got thousands of pots from the dump.

If you have a Lowes near you, go to the area with outdoor pond shapes/liners. They should, my Lowes in Pa have a round black 20 inch deep plastic pot that fits in a whiskey barrel cut in half. The black plastic pot is likely more than 7 gallon though. Sorry, the Pa gardeners site is dead, I am hoping my post helps, though. If you can get some free where Zackey said that's even better.