They come in a parts kit, ive made em with needle nose pliers and a paper clip numerous times... in fact, all but one of my barrels has a paper clip detent...
Its easy, straighten out the paper clip, put the tip of it in the pliers (the very very end) pinch down with the pliers and wrap the clip around 1/2 of the nose of the pliers. Then cut to length and put it in. you may need to bend the end that goes down into the barrel just a tiny bit... too much and the bolt will sheer it off (only happened once to me)
hope this helps
Edit, also, use "small" paper clips because they bend easier and cut easier... the jumbo ones work but arent necessary