In the chat: hamburgermenu>“Encryption info”
This displays the fingerprints.
Is K9 not capable of sending an e-mail with Autocrypt headers in step 1?
A caution; DC does not yet have key rotation. It will reject messages with an Autocrypt header that does not match what it expects, so the trust-on-every-use situation is absent.
Alternately, if your keys are published (most keyring programs and some mailclients let you publish them) you can get them from a WKD bot: