Very few places can guarantee snail free plants so what I've done for years is soak new plants for 20-30 minutes in a mix of 1 part hydrogen peroxide 3% to 3 or 4 parts water. It kills most snails, eggs, algae, and other unwanted hitch hikers.
I've gotten plants from The Planted Tank for sale page. They have a lot of the less commonly found plants. I've used ExtraPlants.com and AquariumPlants.com as well. The only plants I've ever gotten over the years that were absolutely guaranteed snail free are the plants you can buy in those little tubes they sell at Pet Smart.