If you contact them Topeak typically sends you whatever part is required for free. They are very good about it. I always ask about buying the parts and they set them gratis. There is a rebuild kit that has all of the parts that typically wear. They usually are good for a lot of use before a rebuild is needed though. Mine had been used hundreds of times before it got to that point.

When I was clumsy enough to break one (my fault), I contacted them about buying parts and they sent them for free. It did take two tries before I got the right part. Ditto for when I needed another mount to carry it on another bike.

You can email them from their web site. They respond fairly quickly.