Buffalo Trace Distillery has a service for those collect all 8 horse stoppers. If you send them to BT, they will mount them on a barrel stave and return them at no charge.
The most frequent question I receive on this site is how to go about doing this?
While I'm not affiliated with Buffalo Trace, I can help! Sources (i.e. Google) tell me that after you collect all 8 you can contact the BT Gift Shop at 800-654-8471 or 502-696-5926. You can also email them at firstname.lastname@example.org.
Some have speculated if this is a real program or not. I have seen posts on reddit and Facebook confirming it exists along with pictures although I've never picked up the phone and called them myself. Feel free to comment if you have more information or can verify any details.
Finally, I ran across some posts indicating that Buffalo Trace won't accept stoppers that are "new". You can purchase any letter you wish from the gift shop in person or online. I suppose if you have a few new ones you could always age them a bit by putting them in your favorite bottle for a while though. :-)