You really have to go through their authorized dealer network. They do everything they can to not do business directly with regular folk like us.
Goto the Conn-Selmer main home page and use the dealer locator to find an authorized dealer nearest you. Tell them what you want to do. You might run into a problem though. If you tell them the truth they may not want to give you merkatic parts for a non merker matic horn thinking you might try to sell a non merker as a merker. The other problem is if you don't tell the truth they might ask for your serial number of your merker matic.
I'm all for telling the truth and being as respectful as possible.
http://www.conn-selmer.com/dealerlocator/
<Added>I don't know if you can still do this (you could when I was in school)but another option is asking for the specs on the caps. If you get the specs you can have someone in metal shop make a set of caps for you then have them plated at the local jewelery shop.
If you're not in school you can look up a good machinist in your area.
