It's most likely the heater element part of the sensor that's failed as it's a very common complaint for all FI cars these days, but it's not unknown for the feed voltage to have a problem, it heats via 12v supplied by the fuse box.
(but the sensor self generates between 0.1v and 5v back to the ECU)
So a quick check at the connector with a voltmeter should see one wire report battery voltage with the key on.
The front and rear sensors are the same part, but do slightly different jobs, so can be swapped over.
The rear sensor monitors the Cats performace and does not influence fuel trim (but the front one does, that's why a duff one makes the car run like ****e), at worse all the duff heater element sensor will do is give you a Cat performance related code if fitted post Cat.
But this is doubtful as the Cats performance alters due to heat of the exhaust anyway.
There are only a couple of companies that make O2's for everyone, one is NGK (badged NTK) if you really wnat a new one.
sparkplugs.co.uk will sell you one, I've used them in our old Active and a few other cars without issue.
Not sure if it's genuine, but you could try shop4parts for one of those.