Of course you can, from any church supply house. And they don't check ID.
If you're Protestant, it magically happens when you eat them, but if you're Catholic, it's a whole other level of magic. The wafers change when the priest changes them, and stay changed for good.
It was an issue in the Middle Ages, with peasants stealing the host and using it to cure their sick cows. This is technically witchcraft. The Protestant answer was no, it doesn't work that way. The Catholic answer was better locks.