Although I haven't needed to recruit external examiners, I have served as one and have watched colleagues recruit them. Typically, the advisor is working with students in an area related to the their own work, and thus can reach out into their normal professional network to others who work in a related area. Thus, I doubt that a forum for making these types of connections exists---usually, you already have a bunch of colleagues who would be good candidates and who you already have some sort of relationship with.
It sounds like for your case, however, the student is doing something that pushes into a new area where you don't yet have a network. Other people in your institution, however, probably do. I would thus recommend starting by finding out who works in the area at your institution and approaching them: since you are both at the same institution, you are more likely to get a collegial response rather than a "who are you?" failure to respond. Your same-institution colleague, then, can make introductions to good candidates for external reviewers.
If there's nobody working in the area at your institution, though, you'll need to fall back to what you are already doing. I would recommend, however, that the best people to contact are not the luminaries in the field but others in a similar position to yourself: relatively young, and thus more likely to find it valuable for their CV to be an external reviewer.
