The reality is "reputable" growth hackers - aren't sitting around on freelance sites waiting for you to hire them. If they are good, they are either (very) gainfully employed at a reputable company making very good salaries or more likely using our super powers to create multimillion dollar companies. So the bad news is there is no platform where you can hire reputable growth hackers BUT here is what you can do:

You can borrow some of their time on here for just a few dollars a minute and learn what you need to learn to do it yourself (or at least get started) or find courses to learn "growth hacking" yourself. Get the tips, learn the techniques, tons of free resources and sites too.

Unfortunately there are no silver bullets and very seldom magic hires that will suddenly get you growth. It's a painful, iterative long process.

And the term growth hacker is very broad. Some of us can help you with SEO/SEM, email list building, partnerships and PR (I'm heavily specialized in PR).

TL:DR Talk to specialized growth hackers, learn, and apply the techniques yourself.