Honestly, posting on here under the Jobs/Freelance tag can work well. I’d be sure to hint at what the needed component is. Being vague of only saying “I need a custom component” probably won’t entice any devs that much. I know devs like Pragmaflow(@TKOTC) have engaged in creating components off seeing my requests on this forum.
Other than that, NoCodeMonkey has a custom component request on their page and are key contributors to the new marketplace. Complab (who I had a bad experience trying to get a quote) also says they offer custom components.
I tried posting to UpWork in the past and would send candidates to this page to clarify this is in their wheelhouse - Introduction | Adalo. Just remember, you’ll probably be paying hourly for their learning curve on how to build for Adalo.