Need to isolate my ammeter (measuering and powering the unit from same power source)
You need a "floating" power supply:


Could be a mains powered wall-wart or similar. Cheap and easy and connect and go.
Could use an isolated switching power supply powered from any of the AT supplies. This could be an inductor based design or a switched capacitor design as was discussed here recently. Be sure that the isolation provided meets your safety needs for both equipment and people.