Where should I hire great growth marketers?

You're after experienced folk that work or have worked in the same sector as you.

There may be niche job boards for the latter.

If you're hiring someone for Embarque.io maybe try SEO and content marketing related job boards. Look at some of the main publications / destinations in those spaces too as they may have a job section.

Here are a few generic job boards that I have used in the past:

https://weworkremotely.com
https://www.workingnomads.com
https://startup.jobs
https://nodesk.co
https://dynamitejobs.com
https://angel.co