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. It is! I have seen many posts on reddit and Facebook confirming success.
While you're here, take a look around. Warehouse H documents all things Blanton's including many rare bottles and special releases produced for La Maison du Whisky in France, and other releases only available annually elsewhere in Europe.
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. :-)