If you want to use i-TTL, the safest options it to go with Nikon. The cheapest Nikon flash available new would be the Nikon SB-700, or the Nikon SB-600 if you can find it used.
However, if you don't need i-TTL, you can get away with any cheap flash. For example I have a pair of YN-460 speedlights, dirt cheap compared to the Nikon flashes but can still be used as optical slaves. Combining optical slaves with using i-TTL on the SB-900 may not work however, as the slaves can be fooled by the pre-flashes used for measurements. Some flashes have a mode that is supposed to ignore these preflashes and only trigger on the "real" flash, but I can't vouche for the accuracy of these systems, YMMV.
Also note that, as an alternative to using optical slaves, you can use radio triggers. These will have to be purchased separately, and you need one unit per flash, plus one for the camera, but it will give you increased reliability and range. It will also get around the problem of the on camera flash affecting the lighting in the picture even when set to "off", as a pulse from that flash is required in order to trigger any other, i-TTL or not.
Something worth considering is that you could get 2 or even 3 flashes + radio triggers for less than what that SB-900 cost you ...
