Toyota Repair: Oxygen sensor, o2 sensors, toyota dealership

Question
My check engine light came on and the toyota dealership told me that my B1S1 was bad.  They said it's the sensor right against the firewall.  I don't feel like paying 250 dollars just for the part not including labor to the dealership to fix it.  I want to buy the part from an auto parts store and change it myself.  The mechanic at the toyota dealership said that it probably wasn't a good idea to get the part somewhere else, ie. O'Reilly's, AutoZone, etc....  I've changed plenty of O2 sensors on cars for my family and friends, but not sure where or how to change this one.  I have a 2001 Toyota Tacoma, 4x4, single cab, manual trans, 2.7 liter 4 cyl.  Can  you help me out with this?   Thank you very much.  
Ryan

Answer
This sensor is located in the exhaust pipe just ahead of the cat converter held on by two 12mm nuts, easy to replace, whether or not to use OEM or aftermarket is up to you, I have no experience with aftermarket O2 sensors and don't know how reliable they are.