1/4-20 Star Nuts. You can order from Amazon, McMaster-Carr, bike shops....etc. Only like $15 usd.for a pack of 8. Plenty of info on here, just by searching "star nuts for ssp" at the top of this page.
If you only need 1 or 2, maybe other member with few spare star nuts can offer to send them to you. I have 1 or 2 spare somewhere but I am in Canada with cross border shipping issues.