Besides funding non-profits and high-impact for-profits, altruists can pursue less common funding models.
One example of an unusual funding model is the use of certificates of impact. In this model, altruistic work receives some or all of its funding after completion rather than beforehand.
A less formal mechanism for rewarding socially useful work after the fact is the use of prizes and honors for doing good work, such as the Nobel prizes. A more formal mechanism is a social impact bond, in which investors provide up-front funding for services and a donor or government funder commits to paying them back (plus a risk premium) depending on the social good achieved by the project.
Brunt, Liam, Josh Lerner & Tom Nicholas (2012) Inducement prizes and innovation, The journal of industrial economics, vol. 60, pp. 657–696.