Well...no one, Ace or True Value, carries a 10mm X 15mm. The smallest they have is 20mm. To long, for sure.
Dealer is now searching old parts bin plus looking through new parts catalogs for bolt. I would think it would be common on many motorcycles...even today's new ones....but the parts department is not set up to search by dimensions...just part numbers....and no cross reference to a new number for this little bolt.