They're still available new from the dealers, GM/ACDelco part number 15025375. $450 each.
LMC Truck has some cheaper aftermarket options. No idea how they fit compared to factory GM parts, though. Or if LMC will ship to Canada.
Car-Part.com is a junkyard parts inventory checking website that can be useful for finding used stuff, pretty sure they only track inventory in the USA though. There might be something just over the border that you could go get depending on how far North you are.