Any hardware store has it. Don't use fingernail polish remover. Although it is mostly acetone, it often times has other additives that could be undesirable.
I've found acetone in Home Depot, near paint thinners and other solvents.
While we're on the subject, they also have denatured alcohol. Is that the same as the isopropyl alcohol others have suggested here? If not, where would one find that?