I have been looking all over and finally found some nerites at Petsmart. I wish I had found them at a better store, but none of the smaller independent shops had them.
I bought 5 total, 3 zebras, and 2 tigers. They are mostly very small, one I would consider medium. I put 2 of them in my 2.6 betta gallon tank, and 3 in my 4 gallon betta tank. If all goes well and healthy, I plan to move some of them into my other 2 tanks in a few weeks. I put some plastic over the holes, so they should not escape.
My tanks are cycled, and I do 2 50% water changes each week. The additional bioload from the nerites won't require more water changes, will it?