My live feeders go in the sump of the tank, or in one of my display tanks if they're snails. Usually I do frozen with a plastic fork.
That being said I put in 3 astrea snails in the mantis's main display tank, she's only killed 2 of them in the past few weeks. If its snails could probably just keep them with the mantis until it decides it wants to eat.
My other feeding method is going to the LFS and offering to buy anything that looks sick, show up on delivery day, look for stuff that didn't make it, offer to buy it for a dollar and feed the shrimp as long as it's not poisonous. Emerald crabs are like 8-10 bucks each and the LFS is just going to let them rot/throw away so its easy to talk them into it.
Last time I did that they just gave me a bunch of dead free emeralds I froze. Mantis loved them.
Depending on the species gl finding one, you're better off having an LFS order one in for you cause I never seen them for retail. Last one I seen even available (aside from some G. chiragras misidentified as G. glaborus on liveaquaria divers den right now) was about 4 months ago and I bought it, none of my alerts from other places have emailed me and none of my LFS have called, i just cancelled with one of them last week so they didn't keep trying. The wholesalers from LFS are your best bet since their listings are rarely public and LFS rarely order in mantis that take up a whole tank space, you just have to get lucky its the one you wanted.