The first and most important part of finding a great graphic artist is making sure they are skilled in YOUR genre. Some are multi-genre, while others are experts in one specific area.
On my author group, Indie Writers Unite, I have a Yellow Pages document of cover designers that you can get quotes from.
When I wrote, Black Diamond Death, I was still getting my feet wet in the industry, and I went with a talented lady, and I thought the cover turned out good, but then I found someone who took it to the next level. So sometimes it's trial and error, but it's a great feeling when you find that perfect fit.
To point you in the right direction, a good place to start is by looking on my Yellow Page list on Indie Writers Unite.
And here are a few recommendations that are affordable:
FLIP CITY BOOKS - Christine DeMaio Rice
She is quick, sharp, and affordable--and she can do any genre. Tell her I sent you.
ADORKABLE DESIGNS - Jack Wallen
Jack is both a talented writer and designer.
Medium--but still affordable--designers
Author J.A. Konrath uses Carl Graves as his cover artist.
Bob Aulicino has designed covers for J.A. Jance and Lawrence Block.
And here's something really cool - it's a video that shows how a book cover was made. Love it!
Now, as one final item of note - I want to caution you all NOT to pay too much. I believe all the graphic artists I have listed here to be honest and fair, but please be careful. I received quotes from under $100 to almost $1,000 SHEESH!
I believe anything up to about $500 for both an eBook cover and a paperback cover, complete with spine, is acceptable.
Some authors also offer pre-made covers, where they just take a stock photo and slap your title and name on it. It's a good option if you are writing just an eBook novella or novelette that won't be in print. But you need to understand going into something like that--it's a possibility you'll see your cover on another authors book as well. Stock covers can be bought by anyone.