我想買背囊

i think outdoor and eastpak are good,
but I havnt used subcew or xlarge before.
It would be the cheapest if you can buy it when they have discount,
but if you can't, I think you should go to those shops that sell bag (especially student 背囊), they usually sell for a lower price. (I know there is one in 'lok fu', inside the plaza, there is a shop that sells bags.