Per this, their funders include "Sarah Scaife Foundation, the Koch Family Foundations, Lynde and Harry Bradley Foundation, the Walton Family Foundation and the Carthage Foundation."
IHS is associated with George Mason University (whose Mercatus Center is yet another group funded by the Kochs). Charles Koch is chairman of IHS' board. Another board member, Art Pope, is a director of Americans for Prosperity. AFP is yet another Koch-funded group, and they also helped organize various tea parties events.