To obtain their public key, import them from email they have sent to you and that includes it. Alternatively, you can try to discover their public key on a directory.
I gael eu allwedd gyhoeddus, eu mewnforio o e-bost y maent wedi'i anfon atoch ac sy'n ei gynnwys. Fel arall, gallwch geisio darganfod eu allwedd gyhoeddus ar gyfeiriadur.