CTO @Scholars App, founder of @Morpheem, agile coach at a fintech, consulting as PM for several large German media companies, software architect at @Polopoly (a major CMS vendor)
I've found designers in two ways:
* browsing Behance or another portfolio website and contacting designers whose work I liked
* finding a freelancer on Upwork
As always when hiring people you will need to go through quite a few options before you find a good fit. Take a good look at their portfolios before contacting anyone.
On Behance, you may find that many are employed by agencies that might be too expensive (depending on your budget), but you can find independent designers in low-cost countries that are affordable.
Upwork tends to the be the budget option where you can also expect lower quality. There, it may be appropriate to hire a couple of designers for a trial project with a low budget and then make final decision based on actually working with them.