A very cheap thing that you can do is take an old computer keyboard and take the keys off the board and use them for studs. That is what I did. You can repaint them to whatever suits you best, and bam, you have shoulder studs.
Jeremy's right. Specifically, you're looking for old adding machine keys, which are flat on the bottom, not slanted, like today's modern computer keyboard keys.
I recommend filling the inside of the key with Bondo, and setting a hexnut in place. When you run the bolt through your collar and back armor, the nut will secure the bolt.
Although there are some collar bolts sold on eBay from time to time, they are recasts of a TDH member's aluminum studs offered here a couple years ago. SpideyFett and I did a side-by-side comparison once, and got exact dimensions.