The Samsung 840 Pro SSD is a 2.5" 7mm
(Ultraslim) Form Factor drive.
This link seems to suggest that 7mm SSDs use M2-3 (M2 3mm) screws.http://forum.notebookreview.com/solid-state-drives-ssds-flash-storage/570180-intel-320-ssd-7mm-screw-size.html#post7381766
Here's one place where you can buy them. http://www.amazon.com/Machine-Screws-3MM-Pack-100/dp/B009TDQFD0/ref=sr_1_1?s=industrial&ie=UTF8&qid=1367601693&sr=1-1
You can do additional searches if you want to get them somewhere else, or if you want to see if you can get them cheaper.