You can feed them anytime - just size them appropriately for your cham. For a subadult, you probably want snails about 1/2 to 3/4 the size of a dime. For my full grown Jax (who is still a bit small for an adult male), i feed no longer than dime sized snails. He has no problems with them.
As for getting them... because snails are an agricultural pest, their interstate shipping is regulated. Snails in the Mail (http://www.snailsinthemail.com/) may be able to get them to you if they have a shipping partner in your state. That is where I got mine. Contact them first to see if they can get you snails in oregon without a FDA permit.
The snails are rather expensive, but remember that you are only ordering a breeding colony - they breed rather fast and are easy to care for - just keep them fed and moist!
If you decide to get land snails, and raise them, just be warned that they are very messy!! they eat and poop everywhere, and leave trails of slime.
They also multiply like crazy.
I had mine in a old aquarium, and they poop all over the inside walls.
If you are okay with the cleaning, then go for it, but if you dont want to add one more chore to your routeen, forget snails
Thank you for the warning Psychobunny. I'll definitely keep that in mind. That's why I was sorta hoping there would be a place I could buy them online like other feeders, but that doesn't appear to be very common.
For now I'll just stick with crickets and calciworms (which Lola loves as an occasional treat). I'm also attempting to give the calciworms enough time to morph into black soldier flies to try that as a treat for her, too.