Suppose a private key in Dropbox/server. I have tried copying it from Dropbox to iPhone's default clipboard and then to iSSH app but there is no area to paste it in iSSH, very unintuitive user-interface!
How can I get private key to iSSH app?
If you're not very fond of iSSH, I might suggest Prompt as an alternative. I have not encountered a better terminal app for iOS till this date.
To copy your private keys, just connect your device to iTunes and manage the application's files in the Apps tab under File Sharing. You can add your private keys there, and they will be available for authenticating inside the app.
This is not the most user-friendly app. Besides importing the key, you should put MOSH support on for better connection on phone.
Copy your key to Dropbox or any other service to copy the text to iPhone's default clipboard
General Settings > Configure SSH Keys > Import Key... > [Hold finger long, press Paste and insert password]
Add Configuration... > Use Key ON > Key > [Select your just imported key]