What type of electric cable meets code to bury in conduit in Alberta, Canada?

I have a long run (300ft) between the house and an outbuilding. I want to supply a 15 Amp circuit to a receptacle in the outbuilding from a spare breaker slot on my house panel. The distance/load/voltage drop calculators tell me that I am going to have to use #4 (could maybe squeeze by with #6) wire. In order to meet code my plan was to use 4-2 or 6-2 cable buried at 18 inches in PVC conduit. Problem is none of the electrical shops supply 4-2 or 6-2 so I'm thinking there must be a different standard or a more practical solution. What type of insulator meets code in Canada for burial in conduit? Do people normally pull individual conductors? Should I be looking at direct burial instead? If so, what is the standard wire type for that? There is lots of stuff on the web pertaining to US code but I'm having trouble figuring out what is accepted practice in Canada.