Traditional marketing and PR firms are typically selective about the clients they take on; they won’t allow you to retain them unless they believe they can help you. Also, the really good ones are often busy and unavailable. Query early and try to be specific about your goals in using them.
Generally speaking, self-published authors will find it difficult to hire a traditional publicist. Most publicists try to obtain media coverage. Currently, many media outlets won’t consider self-published work because they are overwhelmed with traditionally published titles. Without traditional media coverage, a large chunk of a publicist’s job description is omitted.
Further, publicity doesn’t necessarily sell self-published books. Rather, self-pub authors focus on advertising and promotions through Amazon, giving away ebooks and playing with pricing. Media coverage rarely moves the needle. Yes, there are plenty of publicists who will be happy to take your money and do a great job. Just go in eyes wide open.
Finally: Don’t hire a publicist looking for a one-to-one return in the form of book sales. That rarely happens. Instead, look for a publicist to raise your profile and help secure media coverage you might not have landed on your own. Those media relationships and appearances can have long-term positive effects for years to come.
Here is a 2019 publicist "cheat sheet" put together by people in the industry I trust.