Free: Contests & Raffles.
Go nipple shopping. I use musket caps, but understand that Magnum #11s are just as good or maybe better. The cap should be very, very snug on the nipple. I use keys or the back of my knife to pry/scrape them off the nipple after hunting. If the caps are loose, you're just inviting moisture and other undesirables into your life, not to mention the possibility of a misfire or a non-fire because of poor positioning.
You might try taking the nipple off and taking a file and real easy take a few strokes off the end a few at a time until you take the small diameter off to a little bigger until the cap stays on snug once you get a custom fit you are good to go for years to come.... works with #11 or musket caps... just figure out what works best with your set up