Umm, 160 or 170 is the next sizes down. You would actually be getting less leaf/green plant matter.
I picked up some at Meijers...a department store here in Michigan. Walmart might have them...they're everywhere.
Fdd, I bought about 15 pounds and barely used 5. When I was done shaking it(lol), I dumped it out on a cooking sheet and recovered the dry ice with tongs. When I was done, there was dust everywhere...we need to come up with a better way to capture this dust to keep the work cleaner.
WARNING: Do not tip bucket back over(when done shaking) over work area!