Looking for a newsletter about digital marketing, SEO, PPC ads etc. to advertise on

Perhaps you can look for good ones here: https://www.pigeonnewsletters.com/

Hmm, this one might be a little broad but I know a lot of digital marketers who subscribe: https://fortheinterested.com/

Another one would be: https://marketingexamples.com/ but idk if they're open for sponsorships at this time.