A legally binding contract might do it. Or, there might be some fund that accrues interest over time to pay for the procedure. If neither of those options exist, then I suppose the first people to be revived would be in the name of science ("Let's do it just to prove that we can.") Then, I suppose anyone who's revived after that would be useful from a anthropological/historical perspective. Imagine having firsthand sources from different time periods.
Of course. One possible source could be a family lawyer or descendants. I could imagine this being a viable method within 100 years, but beyond that, it might not be so reliable.
The first method (contracts) is dependent upon people. The second method (money) is dependent upon your currency remaining relevant as well as finding some way of managing the fund. People and money are both fallible, but I included them for completeness. I personally think that the last two reasons are much more likely if we're talking about hundreds (or thousands) of years. At those intervals though, a more relevant question might be, "who would store and maintain the bodies?"
