The keepers are very small parts. I store them in old pill bottles. They are so small that if you drop them in a pile of swarf you may not be able to find them.
To make them you need some 1/4" rod. I made them from cold rolled steel. Chuck the 1/4" rod and drill a 9/64" hole in the end about 3/16" deep. Then cut a slit across the middle of the bar 1/4" deep. This slit should be as thin as possible, however it is not really critical. I used a regular hack saw to cut mine. The difference is that the two pieces will have the same gap when installed on the valve. Next turn a 60 degree bevel with the big part of the bevel at the end of the bar. As you cut the bevel, the two parts of the keeper will fall off. You can fine tune this procedure to make the parts to your liking.
Unless you are very precise in the making of the keepers they should be kept in the original pairs. Don't mix them up.