I presume the question is about one-time project based hire and not a full time resource. Google, as rightly mentioned by you, is definitely one of the ways to search for relevant agencies or vendors. Alternately, you can try one of the following as well:
1. You can try websites like LinkedIn, Quora, Clarity et al to get connected or introduced to a relevant agency or expert per your requirement. LinkedIn could be highly powerful in doing so.

2. Try freelancing websites like Upwork, Zintro, Maven etc to reach out to relevant experts and explore an engagement model.

3. If the project isn't too complex or in case you've 1-2 senior experts then hiring an intern and grooming him/her up could be one of the most cost effective way to get things done.

4. In case you've large capital reserve or are venture backed then you can consider hiring relevant experts. Again, LinkedIn could be the game changer here.

5. In case your business has relevant social presence, especially Facebook, then you can consider running a small campaign to receive relevant resumes.

Let me know if there's anything more I can help you out with. I can connect you to few agencies or experts if you could share the requirement. Happy to connect!!