What ballast should I use for a 90W low pressure sodium lamp?
If I were you I'd be looking for a 90-watt low-pressure-sodium ballast.
That's pretty much the google phrase, though &quot;SOX90 ballast&quot; may also yield some fruit.  Apparently SOX is the slang for this type of tube.
You know, you can get LEDs that emit a single color.  In fact, LEDs were single-color for most of their existence. You'll have trouble beating SOX for efficiency, but LEDs will certainly be better for availability.  The easiest way to get single-color LEDs in a vaguely consumer-buildable form-factor is in 12V LED strips.