Buy them from a Motorrad dealer.
The filaments are allegedly hand wound from purest unobtanium on the inner thighs of virgins whilst sat astride unicorns which makes them worth the premium you will be charged.
Wow, I'm suprised my dealer changed my last one for free!
On my last two GSes the lamps used to last about 15 months.
Now I have LED lights all round, just waiting to see what'll go on these!