When buying new clothes, you will often get a small plastic bag fixed inside the garment containing spare button(s). Don’t just pull it out and store it in a drawer or throw it out. Take a needle and threat and sew the spare buttons to an inside hem, somewhere out of the way which is not going to be annoying on your skin. You don’t need to make it look neat because it is out of sight anyway. You never know, you might need that spare button one day!