Is this a "one sided" seat lug (some oldsters would call it "Dutch") where the bolt screws into a threaded lug eye on one side? If so, you can probably use a regular metric allen-head cap bolt, and it will probably be 6mm. I'd look for stainless steel. If it's a regular pinch bolt, you can find Taiwanese units from any shop who has a QBP catalog and orders from them, typically come in 2 lengths so pick the size you need.