jbadams hit it right.
Depending on the game and style, you may be just fine buying the art, if it fits your game or you know enough to modify it.
But this may not be the case, or you may not have a budget beyond $0. In these cases, you have to take the time to learn, or find a way to make it work by designing your game around it. Many people have taken to learning pixel art. The pipelines for it are very advanced, and there are plenty of tutorials for it. You can get many styles from it too. It is(at least in theory) easier to learn than a more HD style, or using 3d. But the thing people don't get is that it is still hard to make GOOD pixel art.
Another "easy" way is to use vector graphics. Usually there are less color limitations than a pixel style so you may want to still use a sort of palette to keep your game consistent, but it can be easier to modify shapes, etc... than pixel by pixel for some people.
Last thing, which for me is somehow simply easier than 2d is 3d. I don't know why, but for me it is simply easier and less time consuming to create some simple 3d models in Blender, and pre-render animations to sprites. It isn't that I get good pixel art that way, rather the result is simply better than my pixel art results, and takes less time, and more so when you have to make changes.
I should mention though, there are plenty of discussions about the 3d pre-render bit all over, with mixed opinions on cost, time, etc... And for most people it takes more time to get up and started with that method too.
One thing though, there are tutorials around, including an article somewhere on this very site, that talk art "art for programmers." They are great to at least get something started with.