Ebay would the worst place to look for these. If you did get some then you risk them obstructing the airbag deployment. Because the seat side airbags are a safety item you should not do anything that would impede their operation (we hope that they're never needed but if there is a crash then they can be life savers). Not only that but you risk MOT failure or - worse, the insurance refusing to pay out if you had an accident. Try the main dealer and take the hit on the price, or try Halfords and hope they have something for your model. As a last resort find a friend who is an expert (needs to be very good) with needle and thread to cut and hem suitable slots in "ordinary" seat covers.