You can obtain art for your game one of three ways: make it yourself, hire an artist, or find premade art assets on the web.
Pre-made assets are abundantly available on most current game engine marketplaces, individual artist stores, and places like Gumroad, Behance, Dribbble, and more. Many asset packages sit comfortably within indie budgets (some are even free!) and are also used in AAA development to allow artists to focus on more important objects instead of fleeting background items.
Though most asset packages are by default licensed for personal or commercial use, make sure to read the fine print in each package’s ToS and licensing statement.