There was a small local importer who used to bring in Sioux City Sarsaparilla
and other specialty drinks (Stewarts, IBC, etc). He kept getting nailed by Quebec on labelling issues (no French).
This was years ago and I lost track of him - although I do think of him when I see Stewarts in almost every dep in the city.
I haven't seen it personally, but perhaps someone is carrying it.
Of course these brands are popular just south of the border. If you get a chance, I'd suggest trying IBC root beer, likely the best on the market (yet for some reason, they're sticking with saccharin as a sweetener for their diet version). The IBC cream soda is also top notch.
The only sarsaparilla I've seen in this city is Hey Song brand. You can find it in Asian supermarkets. The shop at the southeast corner of St-Laurent and La Gauchetière always has it.
Here's what it looks like: http://en.wikipedia.org/wiki/File:Sarsaparilla-Hey-Song-4294.jpg
You can also find sarsaparilla "sherbet" syrup in South-Asian shops and make your own by adding fizzy water. http://www.rosaaro.com/sarsa.htm