AR-15 mags are 'usually' 30 rd mags pinned to accept only five (5/30 mags). However they may be had in 10 round (5/10) and 20 round (5/20). Any centrefire rifle magazine, with the exception of Lee Enfieid .303 cal 10 rounders and a couple of others, are to be permanently modified or pinned to accept no more than 5 rounds. Having anything over that will get you charged for possessing a prohibited device. Pinning involves blocking the follower so it can't be loaded past a certain point with a device not easily removed. A pop rivet is acceptable, a screw is not.
Having said all that, a centrefire pistol is allowed to have a, maximum, ten round magazine. Rock River Arms produces the LAR-15 Pistol, which is a 'handgun, commonly available in Canada', which may have a capacity of (not more than) ten shots. The magazines for the RRA LAR-15 pistol fit in an AR-15 rifle. The magazines have to be stamped, by the manufacturer, as for the LAR-15 pistol to allow them to hold ten rounds. They can be used in an AR-15 rifle. You can buy them here:
ASC-LAR15-223-10