I used to make my own riced cauliflower, but now I purchase a bag from Trader Joes. I imagine other stores are selling it, too. I put it in a freezer bag, put it in the freezer, and just take out what I need.
I'm nuts about this because it's so versatile—you can put almost anything in it (mushrooms, bell peppers, onions, pesto, lemon sauce) and it tastes good.