If it's models 3060,3080,3180, 3280 or 3380 it takes bag (20) 5026 although bag (20) 5025 will work as well, one being slightly bigger capacity than the other I forget which. Do you have the detachable plastic "bag" collar that slides onto the bags cardboard collar first, then clicks into the mounting inside the vacuum, you need that piece.
Call Sears and ask if they still have 5026 or 5025 available, I think it's no longer available in which case I can send you a couple of bags
BTW it's not the self propelled model you got is it.. my golden quest since mine disappeared last year in the move.