look for apple snails, also called "Mystery" snails.
they come in many different colors, the natural ones are brown, there are also gold, purple, blue, green, and orange varieties, but they are usually not very commonly available.
apple snails are DEFINATELY male OR female, not both, and they cannot switch on you. you need a male and a female for them to mate. they lay a large clutch of pink-colored eggs above the water line, so even if they do mate, the eggs are easy to remove and flush or throw away or whatever. just scrape them off.
i reccomend getting the brown or black apple snails. they tend to be much hardier. i've had both gold and black and the gold ones didn't grow and their shells chipped away and they all died. whereas the black ones, which i got at the same time as the golds, doubled in size and did great with perfect shells.