Why do you need to have the source in a different file with a password? If you want it to be non editable you can try to enter Developer Tab. There open Visual Basic and change the sheets visibility to "Very Hidden" to keep it hidden and this way it will not be visible in Show/Hide Sheets list. Would probably prevent most of the users to see it. And also you can password protect the sheet if you wish.
If you need a seperate file with a password protection you need to open it via a macro as you cannot read data from a pasword protected files than read the data with macro again or Power Qery.