If you are looking to mount your flash to a clamp with a hex receiver such as a Convi Clamp, http://www.kupogrip.com/convi-clamp-1/ or Super Viser Clamp, http://www.kupogrip.com/super-viser-clamp/ and you are using the small plastic stand that comes with your SB-900 flash, http://www.bhphotovideo.com/c/product/570532-REG/Nikon_4951_AS_21_Speedlight_Stand_for.html, the stud you’d need would be the Camera Hex Stud 1/4″-20M, http://www.kupogrip.com/camera-hex-stud-1-4in/ that would thread into the bottom of the plastic speedlight stand. 

 

In addition to the the Hex Stud, a lot of people choose the Tilting Umbrella Adapter with KG000912 & KG001112 studs, http://www.kupogrip.com/tilting-umbrella-adapter-studs/. This gives you the ability to angle your flash and use an umbrella to get a large soft light source from your speedlight, as well as mount it on a light stand.

 

If you want something a bit stronger then the plastic base, you could always pick up the hot shoe adapter as well, http://www.kupogrip.com/hot-shoe-adapter/ which is all metal, is much stronger, and locks positively around the shoe, whereas the speedlight stand is plastic, and the flash is only pressure fit. 

 

I hope I answered your question. If I can help you any further, please let me know!

 

Sincerely,

 

Jason – Your Gear Guru