Not a big deal - these are available everywhere. You can do the "W21/5W Stanley 12 V" search or just search the number 7443 as per rbentnail's post above. On line, you will get dozens of hits from Amazon or major automotive lighting vendors such as Sylvania, Philips or Bosch etc. I think almost any automotive parts place will have them.
To be honest, I don't recall whether the running-light filament was supported in the replacement bulbs I used. If they last half as long as the OEM bulbs, they might just outlast the bike. (I got 180,000 miles out of the original!) I doubt that Yamaha commissioned a specific bulb design for the application but the "Stanley" bulbs might have been the choice of the Yamaha engineers because of a (potentially) more robust design. - or maybe they were just the low bidder?
