I'm curious about silverfish as feeders. They're abundant where I live and they have a fairly long lifespan, not to mention they'll eat just about anything you throw at them.
If you plan on raising either silverfish or firebrats, they need lots of heat and humidity. Doesn't seem to be something many people try due to possible infestation, but same goes for cockroaches, so why not. Firebrats can be bought online if you need a source of some to start breeding, look at various scientific suppliers or other pet forums. Although many mantises are put off by them and will refuse to eat them, so it may pointless.
Your best bet is to try a few and see if you have any luck with your mantises eating them; however, wild caught ones will eat anything including paint and other things that could make your mantis deathly sick... but several members have done it without issues so that is up to you.
I know my mantises refuse to even eat mealworms (hopefully it is just because they are smaller nymphs still). I found out of course after buying 1000 and got a colony going,
Here are three good articles on silverfish and/or firebrats from the forum - topic 1, topic 2, topic 3.