Ahh, the infamous test bar.
I've searched, but can find nowhere to get one.
I'm getting the impression you have to make your own.
But since I can't make straight cuts yet, I wouldn't
know how to do it.

This is why I never do these tests with a cutting tool. Instead, I
use a piece of hardened and ground shafting of known straightness
and roundness. I put this in the chuck and wiggle a little until it
turns as true as I can get it. I then read at two distances from the
chuck with a dial test indicator. Lately I've been using a surplus
Federal Maxxum electronic indicator that reads to .00005" although
that last 5 is pretty iffy. I turn the chuck manually and average the
readings. This thing only puts a couple of grams of pressure on
the bar, so the deflection is pretty small (but it is still there, of
course).
You bypass all the errors caused by heating, built-up edge, cutting
tool wear, surface roughness changes, etc. that you would also have
when actually cutting a bar. Also, you can use the carriage handle
to rock the carriage left and right, and see if the reading changes.
This can detect wear in the bedways or carriage.

Before doing this, I level the bed with the best level available, for
front-back level, at both the headstock and tailstock end. You want to
eliminate as much twist in the bed as you can, as this can foul up
all other measurements.

Jon