You can search profiles or post jobs on sites like Linkedin, Glassdoor, or other job boards in your area. You may also look on freelance sites like fiverr. You can scout and hire talents on Dribbble or Behance.
The best designers I worked with in my career have a good mix of research, strategy, creativity and graphic design. Best if they shine in research and strategy if what you sell will be bought by clients because it’s useful, or shine at creativity and graphic design if they buy it because it’s different/ visually appealing. So you need to figure this one out.
Most designers have portfolios and also do explorations on existing apps. Take a look at their portfolio and discuss with them WHY they did it like this. Let them do the talk 90% of the time and you just listen. Don’t suggest them ideas. Because I’ve seen a lot of designs on Dribbble that were only appealing from a visual perspective but the options there were not really intuitive, useful, or making sense from a customer perspective.
You can also think of a challenge that they can solve in an interview with you. Like “how would you create a financial education app for kids”. Something simple, down to earth and practical. See if the approach the solution from customer persoective, if they ask themselves what are the needs of kids in terms of financial education and if an app is even the best option for this. See their process, if and how they use the needs of the customer to drive the solutioning, and then also if they are actually skilled at designing the solution and working with the tools. How the design actually looks, and if they can deliver files in the proper format to the developer (test this with your developer) etc...
If you have more questions, let me know!