ACE Hardware carries them 10-32 threads 5/8" or 3/4". You can also find fastener suppliers online that carry them. McMaster; Nutty; Fastener Express etc. etc.
I stuck the caps from a Detonator into an Evolution, and it gave it a little of that "Invader-ness." This is gonna make it sound bad, but it sort of mucked up the Evo a bit, like you laid a blanket over the top of your amplifier, but liked how it sounded.
I was always under the assumption that the caps were only to make the Invader look tough, but I'm now convinced they're a crucial ingredient in the tone.
And, yes, I am gonna goof around with replacing some more polepieces with them.
First up is the JB with the Invader's ceramic mags (the JB From Hell ) It's already hella-sweet, and I think the caps will be fun.