If anyone is interested...

At one hardware store I found two packages of 3/8" tubing next to each
other. One was 3/8" inside diameter, the other was 3/8" outside diameter.
The smaller one was marked refrigeration.

At another hardware store I found sweat fittings for the smaller tubing,
which I needed because it is the one that fits the 3/8" compression fittings
I have.

So all is well.