I know a member on here, MGamer2000, has nerite snails, and they are not expensive. I have ordered from him with great results.
Mystery snails are NOT good algae eaters, but they love algae wafers and are super cool to watch! Mine is golf ball sized and awesome, I love her! Contrary to what seems to be a popular belief, mystery snails don't create a lot of waste. I feed mine 1/2 a wafer a day and I hardly ever see any poo.
You can also look into malaysian trumpet snails, they eat everything under the sun, but the bigger they get, the less they come out of the substrate, and if you overfeed, they will reproduce quickly. One nice thing about the nerites is that they only breed in brackish water, well, they lay eggs, but they don't hatch. If you just have a betta in there, you don't have to worry about overfeeding though.
Do you have an algae problem already?