Hey, I usually get my story posters from Kurogane Art Studio and Lazy Graphic Shop
Check out both their work and see who would be a good fit

Kurogane: https://www.asianfanfics.com/story/view/1128555/kurogane-art-studio-graphic-shop-finishing-preparing-for-revamp-for-new-year-graphics-malay-poster-request-posters-postershop-graphicshop-portfolio-art-posterrequest-giveaway-freeposter

Lazy Graphic Shop: https://www.asianfanfics.com/story/view/1233969/lazy-graphic-shop-2pm-angst-bigbang-fantasy-graphic-graphics-infinite-mystery-poster-romance-shinee-superjunior-you-originalcharacter-exo-graphicshop-free-vixx-bts-seventeen-got7-ikon-monstax

I can vouch for both of them, as I keep going back to both these shops ^^