Where can i find a replacement bulb for the nikon d90?

I'm thinking about buying this d90 and it comes with 2 lenses but it needs a replacement bulb for the flash.
Where would I get one?

It would be best to have the flash tube replaced by either Nikon or a local Nikon authorized service center.

People often think they can do camera repairs and then they come on here asking one of two questions:
1) how do I put it back together? OR
2) my camera doesn't work after I "fixed" it, what do I do now?

I think you should think of buying a camera that doesn't need an expensive repair.

Well if you are getting the camera for a steal of a price, don't even bother with the on camera flash tube, Unless… (comment to follow)

On camera flash tubes are quite inadequate, produce undesirable lighting, and wash out exposures many times. An TTL Speed Light that connects to the HOT shoe (such as an NIKON SB800 or SB900) will produce much better lighting and give more flexibility in light placement. The SB800 and SB900 offer light command units that can fire off camera strobes also. You could also purchase a radio frequency command unit to attach to the hot shoe to fire off camera strobes.

Here Is the reason and only reason I see of having the on camera Strobe repaired. The cameras strobe is a light command unit itself, When set up as a command unit it will flash the on camera strobe hundreds of a second before off camera strobes would fire. Yes this was a selling point of the D90 in many peoples opinion.

For the right price there are work arounds that are better than the cost of the repair. Only have NIKON repair center or who they would recommend in your area to do the repair work. The work to repair takes time and is precise.

Here is a link to an authorized Nikon repair center. They offer on-line estimates, but they do not sell parts.

I would strongly advise on not repairing the camera yourself. The flash-tube is not easily replaceable like a light bulb - it is soldered in place.