While you can purchase orthodontic rubber bands from various online and physical retailers, your is the primary and safest source. Using the incorrect size or tension level can negatively impact your treatment progress or cause jaw issues. Where to Buy Replacement Bands
If you run out between appointments, most orthodontists will provide additional packs for free or mail them to you if you call their office. If you must buy them yourself, here are common sources: All About Rubber Bands for Braces | Local Orthodontics