You could use a shallow water dish, but only about 1/8 of an inch deep so that they don't drown.
Keep the substrate wet, but not soaking wet, whether it be coconut coir, soil from outside (pesticide free) or non fertilized potting soil (which most is, so be careful!).
I did what someone on this forum recommended and placed a cucumber slice out...all 36 of my baby snails were on it like magnets!
I originally started out with 4 adult snails, one died a day after I received them (I didn't find the need to complain and ask for another because I knew that they were hermaphrodites and potentially still 3 egg layers). Well another one died about two weeks after that. I'm not sure what the problem was but before it did it laid eggs so that's good at least. I saw the other two mating the other day all over the walls so I'm expecting eggs before too long. It took a little over a week and a half for my first group of eggs to hatch.
Oh and leaves from outside should be fine. I use them after rinsing them off. What does everyone think about introducing isopods into their snail culture? I have thousands of isopods waiting to be seeded into various tanks.
One more thing, I read that this snails are also asexual so they don't even need a partner to reproduce! Imagine that.