If you are using CompleteFTP it is easy to obtain it via the CompleteFTP manager.
Connect to the server with the manager, and go to the settings. SFTP/SCP -> Advanced SFTP/SCP Settings ->RSA Key (or DSA Key).
The ellipsis (...) allows you to export the public key of the server in 3 different formats.
Note - SFTP servers generally have *two* public keys - an RSA key and a DSA key. You can use either one to connect to the server - it depends what the client specifies. So you may want both.