Well there is always Buyairsoft.ca
they do have an M4 in stock that with a little spray paint may be turned into something that could suit your needs. It maybe more costly than the walmart item, but still less than canadian tire. Aside from that the best you can do is wait for walmart to restock, find another walmart or try your hand at trip to or an order from the US.
Please understand that despite your inquiry, this is still an airsoft community, the items in which you speak of, to us are not considerd airsoft. They are lesser grade soft air items, more commonly known to us as poor soft, shit soft crap sot and welfare shit bb flingers. As it stands dont expect to find an overwhelming amount of support. The ones who can aid have already offered their info.