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.
For graphic designs I tend to find the best designer on Upwork & Freelancer,
But If it's just for 1 journal, I'd recommend checking hatchwise.com & 99designs.com - You can start a contest there (hatchwise is much cheaper) and have tons of designer posting different ideas.
Last time I received 150+ logo design idea for a new brand.
I would do the same for a journal or book cover, the investment will be worth it.
Good luck with your project!