This question was recently asked, and I believe there is at least one person on this forum doing JUST that right now. I've never raised mantid as food source, but I would think the best way to do it would be to place mantid eggs in a 22 gallon reptarium or all screen enclosure, with plenty of plastic vines and LOTS of little prey items, such as fruit flies or pinhead crickets. Just keep them all together, making sure they don't crowd too much, and adjust prey item size as they get bigger, moving to flies and larger crickets as they bigger.
Using this method, there will be some canabalism with the mantids, but the only way to avoid that would be to house them each individually (which would cost too much, IMHO).
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Victor Gomez
gomezvi.tripod.com/sdchamkeepers/
gomezvi@yahoo.com