Pete's Superbike generally has the lowest prices for tires even with free shipping from canadasmotorcycle.ca.
Local shops vary but some come within 10-20 bucks of Pete's.
I know Competition Cycle does. It's a bit far for me though so I only go there sporadically but can definitely recommend it.
I haven't found anything on the west side of GTA yet so tend to buy online and mount tires myself.
As for supporting local shops - some are good but some make it difficult to do that - Zdeno's charging DavidR $120 to mount and balance two tires off vehicle? That's nuts. Never liked Zdeno's to begin with, poor customer service and rip off prices on used parts.