Buying steamed buns
Try the frozen foods section of a Chinese supermarket. Mantou are pretty easy to find, but the folded over version used for gua bao can be a bit elusive. I saw some once at the store on Clark just south of La Gauchetière, but they weren't there, last time I went. You might have to get regular mantou and slice them open.
If you don't want frozen, a lot of Chinese pastry shops sell fresh mantou, but most of the time they are already stuff with something. I would do the rounds on La Gauchetière. Something tells me the best place to look would be Jardin de Jade Poon Kai.