The official agency that distributes retail barcodes is GS1. You can contact one of their regional offices using this link:
The official agency that distributes retail barcodes is ISBN, that can be contacted here:
GS1 objects that other entities re-sell their numbers (prefixes and single EAN/UPC barcodes) since number uniqueness can not be assured. You can, however, find several companies that propose such services, their legality is however questionable and the risk using them is ultimately yours.