The two main sites I know folks go to are Ko-fi and VGen (plus I'm on these myself). Vgen in particular has pretty decent on site search functionality so you can look up character illustrations, covers, look through showcases/galleries to get a feel for what a person's artstyle is and then ,if their commissions are open, send a proposal/request after agreeing to their tos. Ko-fi kind of killed their search function recently so you're likely going to have to do more digging through social media to find out who's open, what artstyles people are offering and with caution checking who is or isn't legit not even with ai but also just that you're not getting someone pretending to be another artist since those kinds of scams have been going around a lot lately.
Other options might be checking out who's offering work via the official Tapas discord. I don't frequent it as much but I know I've seen folks post about free/cheap/paid cover commissions you just gotta check dates and see if they've got portfolios to show their work if they don't have any in the posts themselves
Other sites I know of include artistree, artists & clients, and like one or two more whose names I can't recall. But the big overall thing is that prices will vary and be sure to read carefully through artist TOS and be mindful of things like reviews, turnaround time, etc ,etc and of course be kind
