I am not bad about losing the caps from my glow drivers, but I finally (after having some of my batteries for 20ish years) ran out of spare caps for my McDaniel Ni-Starters. I found some replacements at McMaster-Carr:
https://www.mcmaster.com/caps/round-caps-9/ Part # 9753K337 Round Caps for 3/8" to 7/16" OD, 1/2" Inside Height, Red
These fit as the originals, you will have to deal with the red VS orange, as OCD permits (or get the black one for a jaunty contrast - Halloween is coming, you know). A pack of *100* is $5.37, so you will not have this problem twice. I would suggest combining it with another order to save on shipping.
These are exact fit for the McDaniel glow drivers, but I presume that they will fit any of them that twist-on since the diameter is constrained. The clamp types, maybe - the caps are pretty flexible. Or maybe you need the next size up.
Given that I have lost 3 of them, total, since the early 90's, 100 seems a bit overkill. I will carrying them around with me, if you need a few (really need a few spares, don't just grab a handfull...), please hit me up. I won't ship them around, the stamp costs much more than the cap.
Brett