I haven't replace the O2 sensor yet and it's now off, it is set to
shut off after a certain days or something else happened?

Looking for info on the above  - Anyone out there have some ieas where
I could go for these parts and what the cost might be ?

Do you think you would get a better deal if you negotiated one car by
yourself or got together with five others and negotiated five cars with
the same dealer?