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.
Contrary to popular perception, math is filled with character. Numbers often seem to have personality, different fields are characterized as much by their style as by their content, and mathematicians themselves are often more enthusiastic about the beauty of their subject than artists. On 3b1b, the pi creature has come to be a symbol of this unappreciated emotive quality in math, and the plushie pi is your chance to hold a physical reminder of this fact.
We also have a few other ways you can show your love of math and support of the channel in a more tangible way, including a several shirts and posters.