You mean the bolt for the saddle clamp? I would call around for a real hardware store, prob family owned. If you're lucky and can talk to a live wire, they should know at least what you are talking about. Or some auto shop that does Volvo, Mercs, Hondas, etc.
Or here:
https://www.homedepot.com/p/M4-0-7-x...4638/204283605