There are a lot of companies who offer what you're looking for. You can post in Hire a Freelancer if you're looking for an individual. I would suggest that, at a minimum, ensure they can competently demonstrate having completed successful campaigns in the past.
They should have data or screenshots that go back at least 1-2 years. Also, make sure the screenshots have dates on them. I see a lot of ranking screenshots that say 2016 and someone claims they're an expert because they were successful one time, five years ago.