In another life, long ago, I did grant writing. As suggested, ask around universities and other nonprofits to see if they have recommendations. When talking with someone, ask about their experience, grants they have successfuy written for, and some references. If you are applying for a government grant from state or federal resources, you want someone experienced with that, vs. private foundations. Get a very clear contract of payment...set fee for the entire proposal, or by the hour. If hourly, agree to a cap on the hours needed. You should not expect to pay someone only if you win the grant...not professional...and many grants will not allow you to pay from the proceeds. Also, if you are on a very tight deadline and expect the grant writer to drop everything to focus on your project, you may have to pay a bit more for that privelege. If you are expecting the grant writer to do the research to find grants your organization might be eliglble to apply for, that's a whole other task that takes much more time that you'll have to pay for.
Someone from your organization will need to be assigned to work closely with the grant writer. You can't just hand a writer the proposal guidelines and expect them to do it all, especially if your organization does not have a track record with grants. There is a lot of basic boilerplate information about your organization, mission, etc., that must be gathered, as well the details of the specific project you are working on, the measurable outcome goals you expect to reach, other organizations you are collaborating with, budget details...etc., etc. Your organization will need to help get all of that information solidly in place. The grant writer can put it into the correct language and in the correct format within the proposal, but your application will only be strong if the project your organization wants to fund is strong and your organization is credible and and poised to achieve good outcomes if you get the funding. That comes from you, not the grant writer.
Remember that sometimes, however good the grant proposal is, there are other things that can affect a funder's decision. Everyone does not get funded, and it is not always the fault of the grant writer or the strength of the agency's proposal. I sat on a grant review panel for a state health agency once, and spreading the funding around geographically was important, so even though we had several strong proposals from one part of the state, we had to ensure that some funding went to applicants in other parts of the state, even though the proposals and projects were not as compelling.
Good luck! I have always compared grant writing to digging in and doing a major term paper...intensive and time consuming, lots of research, analyzing data, developing your thesis statement and supporting arguements, etc., then pulling it together cohesively in the required format.