What I would mention is that there are certainly cases where a social impact bond can be service-provider-initiated, where a non-profit can recognize that this is something it really wants to do and may contact as.
Equally though, and in many cases, social impact bonds are something that is publicly led, in processes such as requests for proposals that actually delineate exactly these parameters: which population, where, which communities, and why that's a policy priority.
We've seen both processes at work. The more formal RFP is more common so far in the United States than it has been in Canada, thus far.