The reduced form of "a" is "uh" [ə]. Normally, people say "uh dress." More information on the reduced form of common words here: https://tfcs.baruch.cuny.edu/reduced-speech/

I agree with James that ə is more common but to emphasize or correct someone, I might switch to ei. E.g. Did you say you made [ə] mess? No, I said I bought [ei] dress!

Most often we use the "ə" sound to pronounce the article. Although it's possible to hear "ei" in some cases especially when something is being repeated for emphasis.