Funding for 3blue1brown comes mostly from community support through patreon. The patreon community has also been a great source of early feedback on videos before their public release, especially while working on "Essence of" series since I hold off publishing those until the full batch is done.
While patreon is a pledge-per-video model, some people have asked about one-time donations. I am, of course, more than happy accept such contributions (links on this page), but know that the consistency of Patreon has been extremely helpful in scoping out future projects and expansion.
I also run a podcast with Ben Eater and Ben Stenhaug. We talk about mainly about education, but are happy to get sidetracked on other interesting things.
Part of the motive for starting this was that I wanted a more casual medium for engaging with the audience, so this is where I've put things like Q&A sessions.
Want to show your love of math and support of the channel in a more tangible way? Don't worry, we got you, there is a store with some shirts, posters, and coming soon there will be some adorable plushie pi creatures. I have mine already, his name is Randolph, and I love him like my own child.