The saddle on my 08 Specialized Sirrus has started to turn around at the point where the bellows is.

There is a type of washer with pins in it like the pic below which slot into the post and stop it from turning, but the pins are broken so the saddle is not held in place and is able to turn around.

What is this part called? Is it easily replaceable or is there any other way I can fix it?

Thanks

Paul