you can buy them but in 99% of the cases you've already got a good number on your rock (look with a flash light at night) and if you feed them they'll reproduce. they'll make their way to your fuge/sump where they should be safe from any pod eaters and as long as they are kept fed and water quality is up to par they'll continue to breed and keep their population up. i don't run a fuge at all any longer and I've got gazillions of them on/in the rock. my Mandy's eat more then their fair share (they are a bit on the tubby side
)but they reproduce faster then they can consume them.
kc