Salesforce AppExchange Expert. I've built 3 apps on the AppExchange, which integrate with Xero/Quickbooks, Adwords, and charity fundraising platforms. I enjoy helping people who are considering or embarking on the AppExchange journey.
I've led the decisions around architecture, design, UX, pricing, and marketing of three successful Salesforce AppExchange apps (Breadwinner, Daddy Analytics, and Snow Goose). I also understand the nuanced relationship you might have with your Salesforce AE, and can help with relevant listing / security review decisions. I'm available to help you wade through this nebulous process.
The answer isn't that much different from a regular business - you hope your customers can finance you.
But if you are looking for funding and you are a social business - and you are in the UK - I'd start with free courses like this one.
It's not accepting applications at the moment but I believe they will have more courses soon.
As the answers to your question will be very location-specific, I'd approach this as a local challenge. Many social business investment funds are funded in part by governments and they probably don't want to fund a company outside of their jurisdiction.
I would either start a Wordpress Stripe plugin, or if you have some cash, buy one of the existing ones. Then you have a customer base, and a relationship with tons of businesses who have already given money once to make Stripe work well with their website.
Stony surely knows a lot about AppExchange and Salesforce ecosystem. His good experience on AppExchange and sales clearly reflects in his talks and suggestions i.e. how to best position an App, pricing correctly, and licensing models, etc.