What size do I get and from whom? 1/4 x 28 correct? I wanted to get a Ampco to fit these caps...Thank you for your help!
In my book the AMPCO #11 is the best nipple out there...

But! there are a couple of things you need to know....
1. To put the #11 cap on correctly you will need a brass capper like this Traditions. With the capper you will be able to 'press' the cap on to the nipple post expanding the convolutions on the cap creating a near water proof seal. Hunt with it in the rain and snow - should not be a problem. And here is the problem once you have installed the cap correctly - the only way you are getting it off is to shoot it off or pry it off with a knife blade. For that reason alone a lot of people do not like the Teso AMPCO nipple.
2. Track of the Wolf is no longer handling the nipple so Jedediah Starr offers the Teso AMPCO, but the flash hole/Touch hole has been reduced to .029/.030 in diameter. With a #11 mag primer or a RWS #11 - you will often get a hang fire, the 'flash hole' just will not pass enough gas quickly enough. For that reason I drill the hole to .032" and have no problems at all.
http://www.jedediah-starr.com/closeup.asp?cid=76&pid=863Almost forgot to add that you need a .250x28 nipple or a 1/4" x 28 thread pattern