The bulbs are all over the place, look for a 35watt low pressure sodium bulb at like 1000bulbs.com or someplace of that ilk. They should be about $40.
I have one light that is starting to make noise, but I leave it on almost all the time. I've been running it for 10 years.
I actually have almost 8 of these things. People keep giving them to me. lol