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.
Airson an iuchair phoblach aca fhaighinn, ion-phortaich e o phost-d a chuir iad thugad agus sa bheil e. Air neo feuch is faigh greim air an iuchair phoblach aca ann an eòlaire.