Hello; I have had MTS in my tanks for over 50 years. I have found them to be a positive addition to tanks.
Except for breeding setups and QT, all my tanks have been planted. The MTS have not harmed plants for me.
They will burrow thru the substrate and sre effecient at finding excess food and dead organic material. They are mostly out of sight.
The single biggest negative issue can be an excess number. I had that problem in the past myself. I have had such large numbers as to foul pump impellers and have had huge numbers all over tanks. Excess population of MTS is largely due to overfeeding in my experience. Once I determined that it was my excess feeding and made adjustments, the populations have been very reasonable. I find them to be beneficial.
Do a search for MTS on this forum as this has been discussed several times.
I also keep red ramshorn snails in my tanks. They are more prone to be active during the lighted period and thus to some degree help keep algae down on the plants . The rams horn snails will damage some tender plants when they get to around nickle size, but the smaller ones have not proven to do any harm. As with the MTS, the overall population will be related to their food supply. Overfeed and you will have a lot of them. One thing about the ramshorn is that if the population gets large they can be used to fed the fish. I have kept a set of plires handy and use them to crush the shell ( I learned the hard way not to use my fingers). My fish eat them eagerly. The shells of the MTS are a bit too tough for this.
As for picking some up, ask at some fish shops. They have from time to time given them to me. The MTS are livebearers while the ramshorn are egg layers, both will reproduce in a tank so you only need a few to start.
good luck