I can't speak for the sb900 but I have an sb800 and used a generic battery pack with no issues at all. Works just a branded one at a fraction of the cost.
Similar to camera batteries in a sense, the generic batteries often perform just as well as nikon branded ones.
-My Gear:
Nikon D600 -/- Nikon 24-70 f/2.8 -/- Nikon 70-200mm f/2.8 VR -/- Nikon SB800 -/- and a bunch of accessories