I just searched on Summitracing's website as well as on ARP's and it doesn't appear they have a part number for a single stud listed. Maybe give ARP's customer service a call and see if they'll sell you one? I didn't check Milodon's website very thoroughly, but I bet they'd be the same way.
I believe you can buy replacement studs, as far as I remember though, you have to buy them in a 5 pack...Thats how it was when I worked at a race shop.