where do I get a pet garden snail?

Wait until the day after a good rain storm, or water the whole yard and put out a small plastic plate or something with cucumber or zucchini on it. Or sit a cardboard box out in the grass near some place that has like an edging(wood frame around a garden or cement or some such around a porch, they like to be near edges like that in grass). Wait two or three days and then lift up the box, it's highly likely some will attach themselves to it.

As for places that sell them, I don't know...I don't think so. Never seen it here though.

You could always get a small tank(five or more gallons, ten or more for larger species like mystery snails and apple snails), cycle and filter it, add some live plants and get some aquatic snails. Ramshorns come in all kinds of colors and look similar to some types of garden snails, nerite snails are cool and wont breed unless in brackish water. Mystery snails are good too. There are all manner of aquatic snails you can buy or order online. You can look for garden snails online too, like ebay or amazon.

Here's the only one I can find on ebay. Costly for snails, but they are really cool critters!. http://www.ebay.com/itm/Land-Snails-Helix-Garden-S...

I highly doubt it, seeings as you can find them in gardens. I would suggest roaming around your local park with a plastic box, once you find one simply place it in there. Or failing that just go round your friend's house and when they aren't looking pinch one.