Challah bun for burgers
Who's got the best downtown?
Silversteins at 195 McCaul St. has a good egg bun for burgers, not challah strictly speaking.
If you can travel to midtown then Epi Bakery on Bayview has great challah buns, it's what Nota Bene uses for their burger.
Pusateri's on Bay St. carries products from many different bakeries and is also worth a look.