Code PO135 on a Camry V6 is the 02 sensor bank 1 sensor 1.
On a V6 camry its the 02 sensor closest to the firewall (the rear one).
This particular sensor seems to fail more often than the bank 2 sensor in front probably due to the location right above the exhaust and lack of cool airflow behind the engine.
I bought a new O2 sensor from http://www.oxygensensors.com
They offer several versions for California and Non-California emissions specifications.
Check the sticker under the hood of you car to determine CA or Non-CA.
The price almost doubles for CA spec O2 sensors and they arn't interchangable.
To match OE they offer the Denso and Walker brand. I chose Denso just because I like the quality of their spark plugs. Walker is probably fine too.
If you want to go cheap, they also offer a Universal O2 sensor from Walker for $60. The problem with the universal is you'll have to solder your own connector on.
The total was: $94.98
Denso part number: 234-4622
1998 Toyota Camry V6-3.0L Exc. Calif.
Removal and Installation:
I used the bank 2 sensor in the front of the engine as a test to figure out the proper way of removing the connector. I ended up inserting a thin screwdriver into the top of the connector and wiggling it a bit while pulling down on the bottom half. There is a small tab on the right side of the connector that you have to release.
I was able to get to the rear bank 1 sensor from the top of the engine by disconnecting the air intake at the TB, lifting up the air cleaner and then turing it so the resonator moved down out of the way. You might have to disconnect a couple of small hoses in the process so just keep track of them.
I used the same technique that worked in the test to remove the bank 1 sensor connector by inserting a screwdriver in the top and pulling down on the bottom.
Next was the O2 sensor itself. I used a 22mm open end wrench. I tried it cold but it wouldn't budge. Heated the engine up for a minute and I was able to remove it easily. The exhaust manifold heats up quickly so be careful you don't burn yourself.
Installation is just the reverse. Be sure to add the anti-sieze compound to the threads before you install it.
Snap the connector together. Check to see that the wires arn't touching anything hot.
Put all the air intake equipment back. Don't forget any of the small hoses.
Start it up. Clear the ODBII code.
You'll see some smoke initially from the solvent in the anti-sieze compound.