A HeNe laser tube?
A hollow cathode lamp for a spectrophotometer?
Some of the projection lamps are Xenon short-arc, but these are definitely expensive, at least $3500 for a bare bones system from Hammamatsu.
You can run Xenon flashtubes at very low DC currents continuously, just to get a dim blue-white glow.
What color do you want? Do you want a clear tube or not?
What about just a CCFL? I don't know if these run on AC or DC, actually, just that they have the word "cathode" in the name.
Good day!