I bought a bag of assorted rubber bands from a pound shop and there were several hundred varied sizes, colours and thicknesses. However, I'm told that, for things like Crazy Man's Handcuffs, Pinnacle etc, the genuine No 19 rubber bands from the US of A (or from Russ Niedzwiecki’s web site:
http://www.magicbuzz.net/) are the dog's danglies of rubber bands.
I’m sure there were some suggestions yonks ago about the rubber for Pena Coin- it’s Dental Dam and you can use the rubber from a condom as well. (‘Something for the weekend, Sir?’) I’m sure there will be other suggestions in due course!