Unfortunately - I don't think there's a great fit for that sort of question precisely because its not a question.
You've built a thing. The moment you figure you want to use SE sites proper to raise awareness - you're advertising. Even with good intentions, you risk being seen as a spammer.Putting it in your profile is fine, assuming the right people see it.
While there's a precedent for community selected open source ads "selling" that idea to a community would be interesting, and most communities I can think of where you could pitch it don't have ads right now. It would probably want/need a meta post first. Pets would have a good crossover of interest, but not topics. Great outdoors might work. This would of course need SO convinced that running community ads for charitable projects is a good idea, and proving said bona fides for the projects
Essentially, there's no good site to do what you want to do.
(Incidentally, when folks offer to tip me for answers, I tell them to donate to a local dog charity - so you could do some good simply by being awesome - not a grand way to do some good, but hopefully I got some shelter mutt a meal or two).