Quick backstory: I'm a Psychology professor at the University of Toronto, and I'm trying to recreate a low-pressure sodium lamp demo for my students, because the monochromatic light really gets them to understand colour perception in ways that nothing else really can.

So, I'm looking for SOX fixtures that I can (hopefully) plug in to wall power in my lab or lecture hall, and that's become harder than I hoped because, as the community here probably knows better than I do, they've been getting replaced with LEDs. So I thought I'd ask around here - I'm more than happy to purchase fixtures and have my university reimburse me.

From prior experience, it takes a fair bit of power to do this demo for a large lecture hall - in a perfect world, I'd replicate the custom fixtures my doctoral advisor had (which were dual 90W; we used 3 for a 300-student lecture hall), but I'm not sure how feasible that is these days.

There are 589nm monochromatic LEDs available, but like most of on LG, I think LPS is still king . You can run LPS lamps on different equipment such as fluorescent ballasts, if the arc current & voltage are similar. http://lamptech.co.uk/ is a great resource for researching different types of lamp, their history, and electrical requirements. For example, a 35 watt SOX lamp needs an arc voltage & current of 70V and 0.6A respectively (with an ignition voltage & current of 450V and 0.54A respectively). Using a fluorescent ballast (which I think just a bit more common in Canada than LPS ballasts) with a similar output, or wiring several ballasts in parallel to create the desired output may be an option worth considering.
There will be others on this site much more in the know than me that could give you the kind of fluro ballast(s) you need to run a specific size of LPS lamp. I think Workhorse ballasts are a very good place to start.

I think the fixture isn't too hard to source with some custom or in house fabrication. Perhaps someone from one of the engineering departments can help with the fabrication of a two lamp 90W SOX luminaire that fits your needs. The housing shouldn't be that hard to make, or can be adapted from a similarly sized fluorescent luminaire.

The correct ballast for 90W SOX would be the Advance 71A0590, since discontinued although old stock will show up on sites like eBay. However most collectors use a lower cost option, a Fullham WH-5 ballast with 3 of 4 output leads connected runs a 90W SOX satisfactorily and runs off standard 120v power. The lampholder is just a standard B22 holder (Leviton 8806). You can get examples from eBay.

The hardest thing to find would probably be the lamp. I know they show up from time to time on eBay but they're relatively expensive for collectors. Although with some funding, it shouldn't be that hard to get a hold of a handful. I'd aim for getting spare for each socket. The lamps do last for quite a while but since they're no longer made, it would be a good idea to get some spares.

Another source that comes to mind since you are in Toronto are these crosswalk signs. The city of Toronto is slowly retrofitting them over to LED due to lack of easy availability of lamps. The smaller ones use a single 135w lamp, the larger ones use two 135w lamps. I think a couple of decommissioned examples would light up a lecture hall quite nicely. Perhaps one of the electrical contractors that work for the city, Guild Electric comes to mind might be able to provide you with a decommissioned example. 

I have two Alpha-4 lens assemblies that I can rig with an electronic ballast to power 90watt SOX lamps. You can source 90 watt lamps yourself but I can probably supply a pair in a pinch. Plug included for 120volts. Private message me if interested. I'm in Montreal.

Forgot to add that these "Alpha-4" lens assemblies normally fit 135w SOX lamps. They have no ballast compartment, nor a mounting slipfitter. If simple tabletop operation is adequate, with 90 watt lamps facing up, then this is a very simple job.

Although SOX would be the ultimate lamp here, you could consider using clear mercury vapour. It would give slightly different results, but it might be worth considering. You shouldn't have too much trouble finding a high wattage lamp/fixture to give reasonable brightness in a lecture hall. You would, of course, need to make sure to have a clear mercury vapour (not coated nor metal halide) as those types produce too much colour for your demo.