Are you talking about the tail piece where the strings run through, like this?
You use a 1/8 inch rivet with the shaft knocked out, or you can use brass servo hardware for radio controlled planes, etc.
C. B. Gitty Crafter Supply
I build all my guitars by drilling 3/32" holes and inserting brass tubing. 3/32"x .014 brass tubing such as K+S Stock #1268 comes three 12" pieces per pack for a couple of dollars. If you cut this stock to 3/4" pieces they will slip into any wood really smoothly.
They are completely functional, pretty, and really importantly CHEAP too! You can find this type of stock at most any hobby shop and for a couple of bucks it will build many guitars. I suggest cutting it with a saw to get a nice edge. I use a 0.010" hobby saw with a metal mitre box from a hobby shop.
There are many ways of doing this, but I can't think of a much better way off-hand.
Here's something I like to do with 9mm pistol brass. I trim the length and recess it into the wood.
I just put a "tail-fret" right in front of the stringholes.