Home Depot usually sells them.I found a site that sells the forms to make your own, but still looking for a finished product:
http://www.ezytube.com.au/products.html
A fun fact too about them. When building commercial you have to have a testing lab for concrete pours. Every 100 yards of concrete(can change per engineer and specifications of job) well you do cylinders, the concrete gets sampled off the truck by the lab in a wheel barrels they test air pressure, slump(how consistent it is in terms of soupy or too dry), and then place the concrete in a plastic cylinder tube. You make 5 sets usually. A 7 day break, 14 day break, 21, 28 day and sometimes a hold. The lab puts these in a machine that smashes them at each day given above and it tests the Compressive strength. Thusly why some concrete is 2500PSI, 4500PSI all the way up to 100,000PSI. This is how they test it. Well a lot of construction all over the place can make for a lot extra cylinders. Not all of them get broken per day unless asked. So walla they get sold for stuff like house foundation repairs, building walls blah blah.
Yessiree.I have a dozen 12"x12" (one inch deep) forms. Every time I have a concrete pour on the job I take what's left after the lab tests and make my own free paving stones.