Hi,
They are available on Ebay.
Here is a link to see what the prices are. This is for Ebay USA check for where you are.
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1313.TR0.TRC0.H0.Xnikon+sb900+flash+bulb.TRS0&_nkw=nikon+sb900+flash+bulb&_sacat=0