You have to use a genuine Subaru Air/Fuel Ratio sensor. Nothing else will work.
Most aftermarket sensor's are o2 sensor's, which are different from Air/ratio sensors.
You need the part number for the genuine part, you may be able to search it and get it cheaper on Ebay or something. New from SUbaru they are around 650-700 dollars.
Try Evolution R, they have some available, cheaper because they're not in a box with Subaru written on it. you just need the part number.

EDIT: bank 1, sensor 1 is the front Air/Fuel sensor, before the turbo

__________________
Its just the internet dude, its not real.
F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i