For concertina, whether you’ve been playing for decades or are still trying to figure out what kind of concertina you should invest in, your online community resource is concertina.net. Advice, events, heated discussions about arcane subjects–it’s all there.
For diatonic accordion, check out melodeon.net–a wealth of information, including keyboard layouts, teachers, related websites, and an introduction to abc notation.
For piano accordion with a jazz and/or swing focus, check out Dallas Vietty’s Rebel Reed website. Dallas is also a founding member of the fledgling North American Accordion Collaborative, a 501(c)(3) devoted to educational outreach and related initiatives. Membership is free.
A word about YouTube: You can find countless examples of all kinds of concertinas and accordions, including free instruction on just about anything. You can see instruments being made, see them taken apart, see them repaired, see and hear them played. It’s easy to get information overload, a phenomenon we’re all familiar with by now. So by all means, make use of the resource, but leave a trail of breadcrumbs back to the real world.