Typically it is a good idea to first design and program your game using free generic sprites. Once you have the bulk of the game design figured out, then you find an artist to create custom sprites. This way, you know exactly what you need and want for your game. There are plenty of free sprite packs available on itch.io that you can use when your are first designing your game. If you have already gotten the game design and programming done, it would be a good idea to include a link to it when trying to find artists