I'd just make one. Pitch the tent on top of a builder's tarp. Draw round it and then cut out about 10cm inside the line. Or if that's too much DIY, buy a ready-made tarp that's a bit too big and then tuck/fold the edges/corners in.
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.