I think you can buy it @ boardway electronics now on street with no string attached. May be take the next flight at your transit and make a detour to down town will be easier

Immigration --> Airport Express --> Kowloon Station --> Find the Boardway Electronic shop in the mall --> go back to Kowloon Station --> Airport Express --> Terminal 2 for departure --> Airside Train --> Terminal 1

I suggest you to do Terminal 2 as its basically no-man zone in terms of immigration.