I've never experienced this idea of paying a provider 30+ days after the service. I've seen dentists, optometrists, PCPs, and whatever specialists PCPs have referred me to. If they determine that insurance won't cover 100%, I must pay the remainder to get any service at all. Sometimes I pay after the service is done but before leaving the premises; sometimes I must pay before service is rendered.
Are you located in the United States? This hasn't been my experience. The closest I've had is the office manager determining that I would owe a co-pay or co-insurance and be asked to settle that day. It's always an option to decline and ask them to send you a bill in the mail, which usually does come a month later.
What sort of service provider asked you to pre-pay?
So how do I get that 30-day+ float?