I just installed a 20' tower from OWS last month - a little tricky getting started with their directions, but once you get going, it's not too bad.
Anyway, I purchased a small bucket at Tractor supply for about $4 that fit my stones perfectly. If you don't have a tractor supply, any feed store will carry all size buckets for cheap. I thought a 5-gallon one was a big large. For a float to mark my stone, I'm using a small boat bumper ($8 at wal-mart) to mark mine. Not the prettiest to look at, but very functional and durable.
Hope this helps, and be sure to have plenty of patience and beer on hand for the windmill project.

Joe