Thunderbird is finally out of beta so you can get it through the play store or F-Droid. K-9 itself was also updated so it's now basically a K-9 branded version of Thunderbird.
Idk about other sources, but I get my apk directly from GitHub via Obtainium, and my app recently changed from K-9 Mail to Thunderbird. Icon, app name, and app theming changed. This has been in the works for a while but seems like they're officially switched the apk.