Raleigh Nitro MTB - 5-7 speed freewheel conversion - 135mm spacing
You may not have to get a whole rear wheel, you may be able to just replace the freewheel with a 7 speed unit, depending in how much room there is for the wider freewheel. Maybe you will need to add spacers to the drive side and re-dish the wheel to make room.

Five speed freewheel systems were originally used with 120mm drop-out spacing. To get more speeds drop-out spacing increased to 130mm with 8 sprockets but this made the axles prone to break or bend.

If you are going to replace the whole wheel, get one with a modern freehub and 7 speed cassette, instead of the older and much weaker freewheel system.