"Imperial Quality" is one of the many British quality-rating backmarks applied to 1-piece brass flatbuttons made in that country during the early-1800s.
Unfortunately, when a brass 1-piece "plain-front" flatbutton has a backmark saying ONLY the quality-rating, there's no way to connect it to a specific button-manufacturing company or button dealer. All I can tell you about your button's "source" is that its backmark Imperial Quality means it is strictly a British-made button.