I don't want to get involved with shady companies that advertise products that users of the site are unlikely to like. It would be cool to use the advertising space on the site as efficiently and profitably as possible.

I have no idea how to look for advertisers. Maybe there are some agencies that provide services for finding advertising partners?

https://adsterra.com/blog/how-to-sell-advertising/And what about specialists and various ad networks that provide excellent opportunities for finding advertisers to place ads on the site's ad space? I suggest starting with article how to sell advertising to learn more about ad formats, site requirements, and Adsterra ad network services. Hope it will come in handy for you.