I've got a 16 inch (2mW) bare-tube green HeNe available for $160, shipped in the US. It has ~11 inches of plasma exposed. It is different than the MI one here in a couple of ways.
First, it doesn't have those "rings" part way down the tube. There are no shadows from the middle of the tube.
Second, the beam exits on the anode side. This is especially good if you are making a lamp. The beam exits on the side AWAY from the metal in the tube. You can hide the metal part if you want to.
If you need a PSU for it, I have a 110V labby that will run it for $75
Peace,
dave