Local parts warehouses around here stock them; these places are where
the Carquest, Napa, etc stores go when they don't have something in
stock.

Or Rock Auto: http://tinyurl.com/3basq2

scroll eight results down to the Denso part# 2349021.  Ironically,
they are misrepresenting it as an O2 sensor, but the OE part# cross
reference identifies it for an air fuel ratio sensor application.  It
looks like you could save ~$10 on a Standard part, but the extra $
would be worth knowing it's a Denso for sure.

Toyota MDT in MO

After what I posted, does it sound to you like a "universal sensor
from Advanced Auto" is OK?  Can you post the part number and full
description for this part, or a link?

Anyways, assuming you really are replacing the B1S1 (on the rear
facing exaust manifold) here's what I would do...  BTW. your original
post states that this sensor is "real easy" to get to.  I wouldn't
think the average DIYer would consider the B1S1 easy, especially
considering that other sensor (B1S2) *IS* really easy to access.  
That's why I have my doubts you are referring to the B1S1.

Spray down the B1S1 metal base and threaded boss on the manifold with
PB Blaster several times throughout the day, and do so on a cold
engine.  Do this when you know you won't need the car for the whole
day.  Now, get a 22mm or 7/8" combination wrench.  Unplug the sensor
connector (I use a long thin screwdriver to release the lock tab from
underneath the car and then pull on the sensor harness to dislodge the
connector all in one move).  Thread the connector through the box end
of the combination wrench and place the box end over the sensor hex.  
You want the wrench pointing towards the driver's side of the vehicle,
about 10 degrees below horizontal.  You will see that this position
gives you some room to get your hand on the wrench.  You want the
"clearance" of the wrench to point forward (this is a factor of which
side of the box end that you threaded the connector through; if the
wrench is pointing rearward, then thread it the other way - this way
gives me more leverage and room but it may not be that critical).  Get
your best grip on the wrench at the end with one hand, and back up the
wrench with your other hand for stability, then lift yourself off the
ground and your weight should get it to budge loose.  If it starts to
come off freely, then Bob's your uncle.  Back it out slowly and feel
for any resistance.  -or-

If it moves slightly but is still tight, spray the threads again and
tighten it.  Then go back and forth in that arc, alternating turning
the sensor and spraying PB Blaster.  Try to work the arc larger, but
be patient.  Hopefully you can get it out in this manner.  If you do
succeed but it was a fight the whole way, I would suggest getting a
cheap sparkplug chaser tool with 18mmX1.5mm threads.  The cheapest one
I have seen is a double-ended tool with the common 14mm on one side
and 18mm on the other.  Any decent parts store should have it.  
Otherwise, ask for an 18mmX1.5mm tap (good luck) or an O2 sensor
repair tap if that exists.

Toyota MDT in MO

IT IS NOT!  

YES!!

Don
www.donsautomotive.com