I ordered mine from Rockauto.com. I replaced both my primary O2 sensors and they are both the same part number.
I got NTK part#24618 $76 a piece. They shipped fast and the shipping was cheap. Actually I think I paid $10 shipping for priority mail.
here's a discount code you can apply when you check out and get 5% off.
If you haven't done this before (i had not done it before) I highly suggest going to autozone and getting the O2 socket set and a breaker bar. You can get them for a deposit but you get that back when you return them. It would also be nice to have some ratchet extensions and some adapters for different size sockets. I'm glad I got those things before I started this job.