Hello there,
I just got one from New Zealand from trademe.co.nz but the seller has a site here with the mounting rings for some Canon lenses,
http://store.johnthomson.co.nz/home.php?cat=321
I got one for my EF 200f2.8L, he was very good to deal with, he sent me 2 to try out to make sure it was the right one, with a return envelope.
Took me ages to find them though, after my local camera shop quoted me $300!