http://www.ebay.com.au/itm/50mm-x-2m...EAAOSwzhVWq4dC
Try this supplier. I have bought O rings from them in the past and they are quite OK 50mm will stretch quite a bit and 2.0mm should fit.
I bought some 32mm o rings from them to act as focal extenders for eyepieces as all I needed was about another 1 or 2 mm of focuser travel.