Replacement seatpost clamp bolts?

I could hazard a guess and say that the bolt is likely in the 4-6mm range, but to find out for sure, you can get thee to a hardware store (I dunno which country your in) and size it by comparing to other bolts, sizing it with a nut, bolt & thread gauge (a hardware store will have these for sale but you can use one on the shelf for measuring your built without buying the gauge), or ask a salesperson to help you size said bolt. All I can say with certainty is that your bolt is metric. If you live in the US, Ace Hardware stores almost always have a large selection of bolts. FYI, who makes the seat post clamp and/or your bike?

That is a 4ZA Aero seat post clamp (possibly fitted to a Ridley frame). Althought the bolts are standard metric, they are stainless steel.

You can contact 4ZA for spare parts at: 4za.com4za.comOpen ↗ I'm sure they would send out a couple of spare bolts at no charge.

Alternatively, you could go to (assuming you are in the United States) a Menard's, a Lowe's or Home Depot store and find a metric SHCS to fit. Zinc plated, Cad plated or black oxide will do for a quick replacement if they don't stock stainless steel in your size.