No secondary is not the same as primary. Secondary is the O2 after the cat. And the primary is located before the cat.
I just purchased an O2 sensor from oxygensensors.com. They had cheap OEM replacements. I got the Denso.
You could buy the universal O2 sensor they sell. It doesn't come with the little harness clip, so you have to cut your old O2 sensor and crimp the new one in place. It'll only cost you $60, and it works just fine!
For visuals:
With clip:
no clip: