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!
Guess you have considered asking them to prepay. Depending on number of customer you have and monthly amounts, offering an enticing yearly prepaid option can help you raise some cash.
Have you thought about providers better than clients for direct funding?. If you work closely with a provider who understands your business and has a stake in your success, then they can be better positioned than customers to help you out.
You should keep them informed about the day to day growth and depict a rosy picture of your future. Soon they will develop interest in your startup. Wait for the opportunity to pitch, don't be proactive.