You can probably cut the bung peice off a vehicle at a wrecking yard.

Yep, you can salvage one or make a new one from scratch.  I've done both:



Here's how to make your own O2 sensor bung.
I started with a piece of 3/8 steel and marked the outline with a gasket and drilled and tapped some holes (8mm x 1.25):


I used some exhaust studs from the dealer that were left over.
 

Then I started cutting it out with a cutoff wheel on an angle grinder:

After using a grinder and beltsander to smooth it out:


Now that was one bung, but I need two for my 5VZ.  The first one took me about 2-3 hours to make so I decided to try cutting an old one out of my old 3.0 exhaust pipe and using it.  That took about 2-3 hours too, counting replacing the old studs and smoothing it out with a grinder and beltsander too.  

Here's the two side by side, the one I made from scratch will have to be welded in parallel to the pipe since it's not curved to the shape of the pipe: