While "self service" digital advertising platforms like Google Ads & Facebook Ads are great, don't overlook the fact that there are more "old school" advertising opportunities out there online.
Many niche online communities/publications exist that don't necessarily allow you to buy in to their advertising ecosystems through a third party.
Instead, you might need to reach out directly to the webmaster and negotiate a digital advertising deal.
If you've identified an online publication or community (website/blog/YouTube channel/whatever) that has a strong alignment to your target persona and value message, then loo to see if they offer any kind of media/advertising information on their site.
Don't be afraid to reach out directly if you can't see any info either - you might be able to get your message out to a targeted audience at a great price.
Platforms like Google's Display Network have made advertising online much easier ... but there is a big, wide digital world out there - don't miss out on it.
As a side note, it may also be possible to advertise on specific sites via something like Google's Display network - so check that as well (not uncommon to see sites selling ad space to Google and then having their own advertising system as well ... so weigh up the pros and cons of each).