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.

you can direct hire from www.freelancer.com, share your target and goals. you will get the good response of experience peoples.
or
You can direct give request call from Clarity.fm

Right here on clarity by reaching out via message, follwing the growth hacker hashtag paying attention to engagement rates on Twitter and Instagram. Would also look up variations of what growth hacking means at its core to widen your search for strategists who don't list themselves as self proclaimed growth hackers.

The best way is to go to growthhackers.com. It's where most of us growth hackers spend our time during non-working hours :) Here in clarity I'm not sure how many of us are working on growth hacking, but I personally run a CRO and growth agency. Anytime you need to chat, simply let me know.

Contact Vincent Dignan.
Saw him at SXSW 2016.
His company can be found at http://magnific.com/
@vincentdignan

I believe you can get help from Upwork: https://www.upwork.com/hire/growth-hackers/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath

There is roughly 5,000 growth hacker globally. This might surprise you as that sounds too small. Let me be very specific now.

I'm talking about real growth hackers, these are guys who original techniques are rooted in black-hat strategies and have migrated to the gray area. So why does it seem so many growth hackers out there, they focus on white hat techniques that are open to the public anyone can learn and do. The gray area techniques are secrets.

Why secrets, the less people know about them the longer they work for. 99.47% of growth hacks fail for this exact reason. So the more secretive the growth hack the better it works.

So what does this mean to you, your not going to find the best guys on freelancing sites or other expert networks that are widely available.

You need to network, and find the right person by getting to know them. Why, it's because if they are going to use their gray area techniques on your company they have to trust you.

Final note, on gray area, this is legal means, but not well known to the public. These techniques push the edge of many things without breaking them.

I hope this is helpful....