Heres something that I use, and I find that it works real well.
Go down to your local hardware store, clean nipples in hand, and look for the plastic tubing.
I got some silicon tubing that fits snugly over the nipple and will push down enough to cover the whole length. I then cut it off about 1/8 inch above the top of the nipple and use a cigarette lighter to heat it just enough to get it tacky, then just push it down so that it completely envelopes the nipple.
There you have a nice C&B 'snap cap'.
These last about a month until i need to replace them. I practice at least once a day, with the hammer going down on each nipple about 2 dozen times during the practice.
If you want it to last longer, you could cut the tube a little longer so that there is more plastic/silicon between the nipple and hammer when you melt it and push down.