Great question!

1) You can use "pre-selling" to offer an upcoming service or product to existing customers which they will effectively fund by paying for it (while you create it) so that they are also buy it as your customer: https://thefoundation.com/podcast/episode21 https://thefoundation.com/podcast/episode61

2) You can use a crowd funding platform like Kickstarter or IndieGogo.

3) You can ask customers if they'd like to become an investor in your business and offer them to become more like a partner with benefits and compelling reasons to join you.

4) You can ask customers directly what it would take or would interest them in becoming your investor.

Hope that helps!

Tom