As with any project where I hire an artist, I've put together an Artist's Style Guide for Pop & Locke's Last Heist. This is a document I send out to any prospective artists who will work on the project. It lays out the relevant details of the game, including characters, setting, clothes, etc.
A style guide is helpful for three reasons:
1) Gathering and organizing all this reference material forces you to clearly articulate the visual aesthetic for your game. You might have some image in your head, but you need to share that with others and looking at the same document can help.
2) Searching for this reference clues you into a handful of illustrators who you may initially contact for estimates. Even if you can't hire them, they might know others who will be a better fit.
3) Once artists are hired, they won't have to work from a blank slate. Again, using a common source of visual inspiration saves you both time once you start working together.
You can see a previous style guide I put together for Do: Pilgrims of the Flying Temple. Your style guide doesn't need to be so fancy as either of these, but a simple collection of images and text is a great way to start a working relationship with your artists.