These stocks are reliable sources of significant passive income.I don't rely on dividend income yet. But I'd like to be in a solid position to do so when I'm ready to retire.The good news is that plenty of great alternatives pay juicy dividends. Here are three ultra-high-yield dividend stocks I'm buying for decades of passive income.1. Ares CapitalFew stocks offer a dividend as spectacular as Ares Capital (ARCC 0.87%). As a business development company (BDC), Ares must return at least 90% of its income to s ...