1981 kz650 tail lamp

Seems one of the filament on my dual filament brake lamp has burnt out. The one that let's people know I'm stopping in front of them, yeah the important one. I can't seem to find a replacement bulb anywhere. Can anyone direct me to somewhere I can purchase that? Or at least a part number?

You can use a standard automotive bulb with 2 filaments.
I'm using an 1157LL bulb from the auto parts store. LL = long life.
From the diagrams at partzilla they're showing a 27/8W bulb.