Salesforce AppExchange Expert. I've built 5 apps on the AppExchange, which integrate with Xero, Quickbooks, NetSuite, various Payment Providers, and Adwords. I enjoy helping people who are considering or embarking on the AppExchange journey.
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.
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.