FWIW
For on the cheap. You should be able to find a wide assortment, thicknesses and diameters of black, neoprene or nitrile "o" rings in the plumbing section of any large hardware, plumbing or HVAC supply house.
http://www.allstategasket.com/librar...Dash_Sizes.pdf
They range in sizes suitable for leprechauns
up to nuclear subs.
For single use disposable, you can also get rolls of velcro quick tie fasteners. They are about 3/16' to a 1/4" wide and tear off the roll in 6" to 8" lengths. It offers infinite and quick adjust-ability.
http://www.cyberguys.com/product-det...68#page=page-1
I would however caution anyone considering using a cock ring of any type, to only use one that has a quick release, like the the leather ones or one's with the quick release button or velcro.
When you are fully erect the I.D.(inside diameter) of the ring should be only just slightly smaller than the diameter of your thruster, just barely restricting blood flow. As the blood flow is restricted, it continuously builds-up and you can get thicker to the point of ruptured blood vessels and permanent damage. That's why the quick and slip releases that can grow as you grow are recommended.