You can buy them at any store. I took mine out, took it to the store and found one that looked similar...it fit and works perfectly. I have to go out to my bike to grab the size, since I don't know it off the top of my head.
My lens would allow rain to penetrate into the bulb socket and rust the contacts. In order to keep out the water I filled the socket with dielectric grease when I installed all my bulbs.