Exeter bus station still handles a few buses, but if using 55 Tiverton services and 155 they go from oposite bus station by lewisi’s. to get to the coach stops from bus station, at bus station entrance turn left, go about 200 yards up road then turn right and your there, literally 2 mins walk.
Stop 31 on Sidwell Street is the official stop for Megabus and National Express, outside subway, but they stop anywhere along the short bit of road, as 2 or 3 coaches can turn up at once, I imagine if weather bad everyone will go into Subway, as its 24 hours, there is very little shelter at the stops.