300te O2 sensor

[Eliot]

1989 300te.
My mechanic tells me I could use an O2 sensor. Recommends that I do it myself to save costs. He said "Pick one up". Upon inquiry, I find that there are several types (generic and specific) .... inexpensive (relatively) and expensive. What is the 'easiest' way for me to do this, and save $$$? I am a decent mechanic, but lousy when it comes to electrics (I can connect wires, but don't know squat about meters).... Can you be specific about what brand and where...