Where can I get a Bathroom vent that vents from the long side?

Need to replace a bathroom vent/light unit but all in the stores have exhaust port coming from the short side of the unit - I do not want to have to cut into drywall to match the exhaust port - are there any vents that are convertible. (Current exhaust/light unit is rectangular not square and connects to an opening that draws the air to the outside - problem is the opening on any units I have found is on the shorter side of the rectangular unit)HELP