Go to the top of the index page to this forum, and scroll down to the " Member Resources" section. Then scroll down to " articles, charts and links." Click on to " links. That will give you a list of suppliers. Click on them, one at a time, and look through their on-line catalogs. It will take some time. But, you will begin to find out prices, and who has what.
If you don't know what your gun needs- a nipple size, for instance-tell us the brand of the gun, and model, and generally someone here will know the correct size for that kind of thing.
But, IF YOU CALL these suppliers at their 800 numbers( free to you), they will know EXACTLY what size nipple your gun uses, and what size caps you need to buy. They deal with this stuff all the time, and they want to send you the correct accessories for your gun.
There is NO REASON for you to be trying to learn this kind of stuff at some mass market sports store. The Clerks there rarely know their hat from a hole in the wall, about any guns, much less traditional MLers.
Join the NMLRA. Find a Charter club near you, and meet the shooters there. They will happily help you learn not only all the answers to your questions, but will steer you to local dealers who have the kind of things you want, or need.