Finding a book designer is as easy as Googling really. You can find a book cover design for as little as $5 on Fiverr.com or you can hire a really great designer for thousands of dollars.
Budget is certainly a concern, but equally important (and the piece that many don't see) is making sure the designer matches your style. Every good designer will have a portfolio and you need to take the time to look at what they've done in the past to decide if they can do your job.
I also HIGHLY recommend you interview them to see if you will work well together.
But before you do, do your homework first...
See designers learn through visuals so explaining with words what you want and what you're looking for to someone who sees in pictures means that something always gets lost in translation.
Find your aesthetic first...
What does that mean? Go to a book store and find book covers and interiors that you like. Be sure to look in your genre of book to get an idea of what works for YOU. Spend some time on Amazon looking at covers. Or start a Pinterest Board with books and designs you love. My Pinterest Board for design is here if you're looking for an example.
That way you know when you're looking at someone's portfolio if their design style matches yours AND you'll be able to explain what you're looking for to them in a way that they can see and really understand if they can deliver what YOU want.
Your design is really important when it comes to creating a quality piece of work so take the time to first figure out what you want and then go out and find the person who can deliver exactly that.