Taillight bulb

My rear light went out the other night but the brake light still works Today being Sunday I was unable to have a bike shop open and I went to 2 or three auto parts stores No one could help me with the bulb, Ok the question is... Where do I get the bulb for the rear light and is there an equivilent bulb on the market like 1157 etc.market meaning other than Triumph parts
Thanks in advance
Bigfoot

The R3 tail light bulb is a common 1157.

The R3 tail light bulb is a common 1157.