Iowa Valley Carriage carries them, as does Carriage Driving Essentials in CA and Driving Essentials back east. You can also sometimes find them masquerading as drawer pulls in the local hardware store!
Watch the edges though, I notice the ones at Tractor Supply are a bit sharp on the corners and I'd worry that could mark or damage my harness strap over time. The ones from the carriage dealers have rounded edges where they screw down.
Leia