Customers? Various non-angel funds? Personal savings? Credit cards? Crowdfunding? Debt? These are all the approaches people have used.

I'm not saying that if you can get it from an Angel willing to part with their money you should, but most angels get burned out / burned pretty fast. They should realize that most likely they'll be throwing away their money and never see a return. If they still want to give it to you, then go for it. But there are other ways.

Personally, getting it from customers is the best form of capital. You're actually validating your business and building something of value.

That's why it's called "building a business" and not "hustling a couple dentists out of a few million dollars."