Where could I hire artists? How much would I expect to pay?
If you are actually serious, simply type your first sentence into the google search bar.
The price will depends on how much assets you want them to make.
Just to be clear, following pricing will be in United States dollars. And this is 2D pricing.
Typically, an unique 32px pixelart character with several standard animations will cost you around $300 (cheapest using russian/potato/huehue slaves) to $1000. More animations or more detailed animations will be far more costly.
Tilesets and backgrounds are charged differently and you will pay around $50 to $500 per background, depending on how detailed you wanted it to, native screen resolution and the artstyle you requested it to be in.
Of course, as with everything else, try to negotiate if you are commissioning a lot of contents from a single artist to get discounts for bulk commission.
How much you will be spending for visual will depends on your willingness to create the art yourself, your desired artstyle for the game, game native screen resolution, number of characters, number of "stages" and the gameplay visual requirement.
Regardless, good luck and do your best.