I would say contact the manufacturer. Most retail stores will not sell parts like this (and if they do, it's probably a store that will be something like a kitchen specialty store--aka, a little more pricey), so the best way to go is to contact them directly so that they can get it for you (and, a lot of times, they can get it for you at a cheaper price, or, if you're under warranty, sometimes for FREE!!).

Their contact number should be either stamped somewhere on the machine (usually on the bottom--might be stamped directly into the plastic, so make sure you have it in really good light when trying to read) or in the product manual (usually near the back of the book).
Good luck! :)