Bobs 300

I just bought this car and after three weeks ths Bank two sensor 1 went out(stopped working ) and I want to replace it --I went to Toyota and they want almost 360 $ to replace it-I spoke with a friend and he volunteered to help put in a new one --must I go to toyota to buy one or are there good aftermarket ones out there--what websites?--what brands?--Thanks to all responders--the car is 2001 ES 300 w/63000miles