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.

http://westminster.impacthub.net/impact-investment-ready/

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.