One possibility you have is to find a hiking club. There are some all over the world and that's where you'll probably find the most involved hikers, those ready to invest money on long hiking trips, far from home. Most of these clubs have online forums or some sort of ad board where people would post any climbing partner search ad. They also organise these long hiking trips.
The problem is still that the trip you seem to have in mind requires a lot of preparation and money, so planning it several months in advance is probably required.
I know there are a lot of club sections part of the French Alpine Club, but they do not really communicate together for forming teams. There is also a Alpine Club of Canada and I found that the American Alpine Club has a "Member Share" program whose purpose is exactly to find a climbing partner.
You should try to contact one of these clubs and see if they have such climbing partner ad board, or organise such trips.
Contacting directly the national park to ask if hiking clubs organise such trips might also work.