Order them online and have them waiting at the nearest store. I bought over a hundred last time at that price. In my larger room, square buckets would be impossible to turn. The advantage of round buckets to me is the ability to spin them in a crammed room. If you put square pots directly next to each other, you will have to pick it up to rotate it each day or whatever. The ones in the center of a group of square pots are often hard to get out without first moving a few beside it so you can get your fingers around the lip. I rotate all my plants a quarter turn each day so doing all that is not realistic.
I use 2 5gallon buckets per plant. The bottom one acts as a run-off catcher and the top bucket can spin very easily within the bottom bucket, even with a heavy plant inside it.
-Dude'r