Well, let's look at how things currently are. Individuals (or the companies they work for, or government programs such as medicare/medicaid/tricare/healthy families) pay insurance premiums and co-pays. Often these insurance companies are for-profit - this profit is inherently money going towards something other than actually paying for healthcare. These insurance companies pay the healthcare providers a certain amount when services are used, which often doesn't cover the actual cost. The hospital then attempt to recoup these costs by increasing costs for self-pay patients, who are often self-pay specifically because they're poor and can't afford insurance, so they can't pay either, and end up defaulting or getting charity care.
So "Where is the money coming from?" is a question that applies to our current system.
Now personally, I'm very healthy, and get no more than basic preventative care. I pay thousands of dollars a year for (laughable) insurance that is... going towards paying for someone else's healthcare! That's what our current system is! It cracks me up when people complain about the idea of paying for someone else's healthcare, because insurance is not a private medical savings account - either they're already paying for someone else's healthcare, or someone else is subsidizing theirs.
Now, with universal healthcare, the thousands I'm spending a year on insurance (and co-pays, etc.) go towards taxes instead. I'm cool with that, because I'm no longer paying for the insurance. People who are paying taxes (which I'd guess is a much larger percentage than those who currently pay for private insurance) all pay into this system. Maybe the poorer people aren't paying as much, but they're paying something, whereas currently, between not being able to afford an insurance policy or hospital bills, they are not.
These people who were previously uninsured or inadequately insured get more routine/preventative care and early treatment, which is relatively cheap. and expensive catastrophic care (that they can't pay for) is avoided. People with illness/injury are more likely to get adequate treatment instead of suffering from permanent disability, allowing them to go out and work and pay into the system and avoid disability and welfare payments.
I realize that this may be a simplistic view of things. But the idea that the money isn't there just doesn't seem right to me. It's there - it's just a question of managing and distributing it differently.