You almost certainly won't find one, apart from as part of a whole tent, maybe s/h. 'Spare parts' are a pretty rare thing for far more expensive tents, and generally only whilst tent still in production!
Friend used a bin bag for years on his tent when the cover went missing! If you are passionate about the tent, a DIY or even a tent repairer could knock up a suitable replacement.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.