Microplay might have some older carts but try some online ads You'll get a better deal if you buy from a private sale. Even asking someone here if they have a SNES for sale might get you a good deal.
Years ago, Flea Markets used to have at least one booth full of retro games/consoles. I'm thinking of Dixie Mall in Mississauga, not sure where you live. The flea market was only open on the weekends though.
thanks for all the replies folks, i managed to pick up one of the fc twin systems and mario 3, super mario world at gamcenter and found a cart of magic sword the other day in montreal. i'm a happy camper now