You can look on freelancer websites and marketplaces. Some of these are specialist sites, e.g. Toptal is for UX/UI people.
- Upwork | Hire Freelancers. Make things happen.
- Clarity — On Demand Business Advice allows you to ask questions first and then hire the people who answer. You can buy advice over the phone by the minute. It’s a good way to get quick answers.
- Expert360 | Freelance Management Software
- Toptal – Hire Freelance Talent from the Top 3%
- Log In or Sign Up Linked In will allow you to search for people with those skills in your region.
After you have chosen the marketplace and the skillset you want; you will need to be expert in how to brief an agency, how to write an agency pitch and ways to select from a range of candidates for your work. That’s the subject of another question!