This depends a great deal on how much food is available in the tank for them. If there is an abundence of algae or the tank is overfeed, like any of the other snails they'll multiply rapidly. However, if the tank is fed minimally and has little to no algae, you may not notice much of a population increase.
MTS generally hitchhike in on plants and are considered by most LFS to be pests. Often times they'll give them to you for free or a minimal charge. If you can't find them locally a post in the Barter/Trade forum will likely find someone willing to share.
There are a few people who post them for sale. Normally it's just "pay shipping and get these things out of here". Even a small charge is well worth it. I have them in all my tanks. Neat to watch too.