They are called chicago screws. At least that what I have always called them. I usually don't buy things with them because they fall out to easily and if I do I punch an extra hole and run a leather strip through or use waxed thread to stitch it togeather. You can probably google "chicago screw" and find them. I use a clip on the ends of my reins so I can remove them easier. They sell those at most tack places. Seems I am always modifying my tack bacause no one makes everything just right
If you don't have a local leather place you might look on line. Tandy is a good source. http://www.tandyleatherfactory.com/index.asp
Looks like they call them screw posts.