Gun auctions are good places to look for old mil ball. The previous owner was invariably a collector who would pick up milsurp when he could for his rifles. The last auction I attended in Red Deer had loads of 1943 DI Mk VII Ball (Dominion Industries, Montreal). 1943 DI is boxer primed, so I get to fire 1943 ball out of my 1943 Long Branch. I keep the casings for reloading.
Auctions and Gun Shows may also have a lot of old Dominion/Imperial/IVI commercial ammo in 150 grn, 180 grn and even 215 grn. Even if you have no use for the bullets, pull them (do NOT throw them away, especially the 215 grainers), dump the powder, deprime and and reload with whatever you want to do Service Rifle with.
The Canadian Rangers is a growth industry. They get a lot of 303 made by IVI, both in 215 grn (normal issue) and 174 grn for local and CFSAC type matches. Once the remaining Long Branch and Fazakerly rifles are no more (though they may well run out of magazines first) new bolt action rifles will be acquired - and it won't be in .303.