Any suggestions would help
 

I have a 150cc scooter gy6 engine, without the wiring harness. Can anyone please suggest what and where from I can get the right one?

Only manufacturer. If yours is a roketa, contact roketa usa.
If yours is TaoTao, contact them.

Man In The Box scooter tuning company used to sell a GY6 wiring looms for swaps. I remember the looms being fairly priced for GY6 motors but almost 4x the price for a Zuma 2-stroke motor into a Honda Ruckus.

So try Man In The Box. Also check Scooter Swap Shop.