In the past I have made these from long 5/16" - 18 NC bolts. I cut the heads square and then turned the ends on the lathe so they looked right. Then welded them to the seat track.
Lately a guy started selling near exact copies of the originals on eBay. Apparently he had a bunch of them made on a screw machine. He supplies them with 5/16" PAL nuts so you don't need to do any welding unless you just want to.
Here's a picture of the broken studs. They are swaged in so they come right out with a hammer and punch.