Sports betting in America is tough. If you want access to any of those top books you're going to need to know somebody. To get a credit account at any sportsbook you need to know an agent. This is the person you settle-up with. To get an account at a book that doesn't take Americans you can always team up with someone not in the US. They can get an account and you can relay your bets through them. I never said this was easy.
To put it in terms that AP's around here would understand, there are multiple facets to being a successful AP. The first is knowing how to play. The second, and more important, is to know how to get the money down. You can count cards better than a computer, but if you have no place to play a winning game, you'll have trouble making a living at it. The same is true with sports betting. Acquiring accounts, betting money without moving the line, moving money, longevity with good accounts, and getting paid are all part of the Art of Sports Betting rather than the Science of Sports Betting.
Maybe some day with legalization, there will be sportsbooks all over the place like there are in the UK now. Maybe these sportsbooks won't all be controlled by an oligopoly of companies that refuse to take on risk and move in lock-step with each other. Maybe some day the sports leagues will embrace the idea of betting exchanges and the world will be able to bet with a 2% commission, of which the leagues will get a cut. In those scenarios, getting money down will become remarkably easy. The market will become more efficient of course, but sports betting will be more of a science. Until then, making decent money betting sports will continue to depend on your artistic ability.