I could not imagine a worse combination for target shooting than a Sten gun and airsoft.
Airsoft is not accurate at the best of times. Sten guns are awkward to hold, aim and use.
If you want a WWII AEG get the Thompson from Canadian Tire.
If you really must have a Sten then go through the steps in the FAQ to be age verified and you will get access to a closed section of this site where the "where to buy" details can be discussed.
Airsoft, where nothing is hurt but feelings.