Ways to isolate DC power source
I don't agree that you need to be an expert in medical device design to accomplish this task, but you do need to do some research. 


Get a copy of IEC60601 and study it. There is a lot more to this type of project than just isolating the patient from the computer.
You're going to need to treat your USB connection as if it were mains. Find a transformer that was designed to IEC60601, and matching driver. Design from there.
Find a testing house that does IEC60601 compliance testing. They will test your device to the standards, and give you a 180+ page report.
Go through the report, fix the problems, and test again. 


Do not skimp on compliance testing. It will save your bacon.