*Here are the exact ones that I used, purchased in Spain. They have a diameter of 8mm and a thickness of 3mm. They are super strong and must be used with caution because two magnets can easily pinch your fingers and the magnets are a dangerous choking hazard, so you must keep them out of reach from your children.
Once the magnets have been sewn inside, they aren't strong enough to pinch and they are no longer a choking hazard. These magnets are so strong that they will snap together if they are less than 10cm apart from each other or other magnetic surfaces.
These appear to be the same type of magnet but shop around and see what you like. They don't have to be round, I've seen rectangle ones that look like they could work well too.