You can get them from a bike shop.
Alternatively you can use car 60/55w ones by bending back one of the tabs. I've been running them for 4 years without a problem, though some people have had melted contacts in the switchgear due to the extra powah.
It's also not great for battery life so I run a battery tender, but you should have one of those anyway
A good all in one solution if you have the loots is Eclipze's upgrade kit.
They come highly recommended.