Like many authors, my ideas behind the book series that would consume my life came significantly before my ability and focus to write it.
If you've read my bio, you know I'm pretty strong on the whole science and engineering front, and of course I regularly try my hand at both technical writing and fiction.
Of course, everyone has to have an Achille's heel, and mine is in the arts. I'm terrible at music, and keep having an attempt to learn a musical instrument (I suspect my true failing is not dedicating enough time to it), but I also I struggle with drawing and graphical design.
This is obviously a bit of an issue when you need a book cover!
If you have a more mainstream book, there are places where you can shop for pre-done book covers. For my 'girl on the Moon' stories, I was never going to just buy something off the peg.
I asked a friend of mine about suggestions of where to look for an artist who could bring my book cover to life. I just want to cover one item upfront though, and it's that artists provide a service, so be prepared to pay for that service. Don't go looking for commissions but asking the artist to do it for free but 'they'll get exposure'. There have been a number of people pulling this over the years, and you might be cash strapped (honestly, every corner of the writing process seems to involve paying out more money), but this is your passion project, not some else's. And exposure doesn't pay the bills.
Also, it helps to be clear that you're planning to use this for a book cover, as it might cost a little more. This is because you're using it for promotional/business over just a personal piece.
There is a whole community of artists looking for commissions, try searching for 'commissions open' which most artists will use to indicate they're open for new work. Review their past work and see if it fits with your vision for your book.
Most artists will have a price list, usually for character illustrations. They might be interested in a full cover illustration too.
Deviant Art
There is a great forum on Deviant Art called 'commission an artist' where… well, I think you can guess the rest.
There you're able to browse artist portfolios and find someone whose artwork matches your vision. Some artists will list material they'll draw and types of work they'll not do. It's best to respect those boundaries, otherwise you'll get a reputation that'll be difficult to shake. An example of those boundaries might be someone chooses to work only in fantasy and avoid characters who are overly muscular, so asking them to draw a sci-fi version of Arnold Schwarzenegger is disrespectful.
As I've said, most artists are really clear about prices, but please avoid heckling on prices, it's a way to get the wrong kind of attention. If they're too much for you, move on.
It was through Deviant Art that I connected with the fantastic Emilia Harasim who produced my artwork.
Upwork
Upwork allows you to post a job advert for creating a book cover, which artists can respond to.
I've not heard as many stories about this route, but it's worth looking into.
Maybe you'll get lucky!
You can always try your luck at somewhere like PlaceIt and look at what premade book covers are available.
It's also worth reading the following article: