Are you talking about the metallic mount that contains the contact pins and is screwed to the rear of the lens?
I think its best you contact Canon first- they sell many of their user-serviceable parts for very reasonable prices, before going the ebay route.
By the way, make sure to use Japanese industrial standard screwdrivers- these aren't standard Philips heads. I almost stripped a screw- luckily I noticed before I started.
Mind you, if any contact is broken below the mount then you might either have to buy more parts or need to get it serviced professionally altogether. So you might want to take the mount off first and see if there is any deeper damage before ordering a replacement mount. My 2c. But mount replacement by itself is quite easy.