Do you mean online stores in general? Because the last time I checked, there were thousands.
Which Ultrafire light do you have?
Also, would you like a LED drop-in instead of a bulb?
Edit: I see that you have a WF501B.
So for a lower priced replacement bulb that works with 2 Li-ion batteries,
these and
these should work. There are also many others. If they(the replacement) fit in a P60 accepting light and can take 8.4v, they should work.
LED drop-ins might also be another good choice. I'd recommend
this or
this one because they are single mode and have good output.