I don't think the Bibbidi Bobbidi Boutique sells accessories separately. You can probably get hair clips and maybe tiaras in the main shops onboard, but you wouldn't know until you sailed, so buying in advance would be a better option.
If you happen to be onboard when the BBB is open and not busy, you could stop by and see if they will give you some quick pixie dust. My (20-year-old) daughter did that once, and the Fairy Godmothers in Training were happy to have something to do. Obviously, this won't work if they have actual customers, though.
If you happen to be onboard when the BBB is open and not busy, you could stop by and see if they will give you some quick pixie dust. My (20-year-old) daughter did that once, and the Fairy Godmothers in Training were happy to have something to do. Obviously, this won't work if they have actual customers, though.