Beware! Adapting a screenplay into a graphic novel requires a lot of structural and narrative reconsideration, time and labor, and will cost a pretty penny. Also, there is a trend the last decade of writers trying to turn their unsold screenplay into a graphic novel and, most times, are unsuccessful since the story wasn't originally conceived for the comix medium.
The good news is that you are willing to co-own the concept with the artist, as you seem to understand and respect that the artist is just as much the creator as the writer. You would be surprised how many writers DON'T believe that.
I'm a veteran cartoonist, so, I know some of the best in the biz (and occasionally meet/greet the new kids on the block who might seize this opportunity as their first major work). Any artist I could possibly reach out to would need to know certain important aspects of the job before seriously considering collaborating and dedicating a year or so of their life to any given graphic novel. And, since I'm not a proper agent, there are some questions that need to be answered before I could even search and recommend a sound candidate or two. Questions like:
1] How many pages is your screenplay (I imagine 90pp - 120pp) and how many pages do you imagine your graphic novel to be? The standard is between 120pp and 200pp.
2] Do you have publishers lined up ready to consider your graphic novel or are you going to self-publish?
3] If you're going to self-publish, do you have a distributor and/or have you looked into securing one? Have you considered digital distribution, too?
4] If/when the time comes to promote the book, will you pay for a publicist to market/brand/etc.?
5] Do you currently read comix on a regular basis? Are you aware of what trends, what's popular, and what's cool (three VERY different kinds of comix to consider as you put forth your concept into the mix)? In other words, do you have a sense of what sells?
Alas, unless you're making the next WATCHMEN or THE DARK KNIGHT RETURNS, your graphic novel will probably not break even. I've worked on several critically-acclaimed graphic novels that paid me a good advance because they were financed by DC Comics which is under-written by Time Warner (who has money to hemorrhage) and they have yet to break even. I have yet to receive a royalty check for well-regarded work I've done with Harvey Pekar (famous for American Splendor) and Jonathan Ames (famous for many novels and essay collections, including HBO's "Bored To Death"). I occasionally get a few bucks here and there for a Batman or Spider-man comic I drew but never anything I've done for creator-owned comix. Welcome to my world.
So, with all this in mind, the artists I could help you seek and hire couldn't do the job for less than $300 per page. And, that's for black & white art ONLY (before lettering and coloring/etc). If you're willing to adapt your own screenplay into a comix script (because that's a whole other job, entirely), that would help knock down the page rate but if you didn't feel confident enough in doing that, a veteran cartoonist could do that -or- I know a few writers who could and that would cost, too. Naturally, rights and royalties would be split 50/50 (but you could work it into your deal that you recoup your advance monies and then split royalties evenly once costs were recouped). But, the artist would have to live on said funds in order to produce the work.
I don't write this to scare you or dampen your dreams but to give you a better sense of what it takes to produce a graphic novel. I hope this helps.