These people may have what you want
I purchased several of this item when upgrading my first rail kit.
I am not saying that is the screw you need, you'll need to determine the thread etc
haha, two clicks later i've found this one - it looks perfect http://workholdingcomponents.mortonmachine.com/viewitems/thumb-screws-metric-thumb-screws/metric-precision-stainless-steel-thumb-screws?
It looks like you're new here. If you want to get involved, click one of these buttons!