The Nitto "Mark's rack" sold by Rivendell is made for mounting to fork blades (whether to braze-ons or with clamps.

A less-expensive alternative is the Velo-Orange randonneur rack.

Also, if your intent is to support a bag (as opposed to hang panniers), mini-racks of various sorts and connections show up on eBay. I bought one from Hilary Stone recently that mounts to the brake bolt of centerpull brakes.

Neal