From a Canadian Perspective:
The original TurboGrafx-16 was being sold pretty much everywhere: Radio Shack, Toys R Us, Compucentre, Consumer's Distributing (I worked there, it was a catalogue shopping where most of the store was a warehouse, with a small display/store front.) I think Sears, Eatons and Zellers probably too... (I don't remember of K-Mart was still around back then?)
By the time the Duo came around, I think it was narrowed down to Compucentre (but I couldn't find the newes releases) and Radio Shack. I think Toys R Us might have had actual Duos now that I think of it; I'm having a faint memory of seeing pouches with the DUO sheets of paper in them... But I don't seem to remember TRU having any software titles (from the SuperCD era). I don't think "Electronic Boutique" had anything for TG-16's when they showed up here.
I can't forget about Microplay. But I think they only really carried a handful of used systems and games.