Personally, I don't like microbeads - they go everywhere and are difficult to clean up, so I don't make a point to look for them. BUT I have seen them at Hobby Lobby when I was looking for something else - yes, they were in the back of the store in packages with the other beads.
I am guessing any place that sells beads (Michaels, AC Moore, Walmart, etc.) will carry these as well.
Good luck!