polycarbonate replacement

Does anyone know a good place to get small amounts of polycarbonate sheeting for greenhouses? I have a small (6'x2') polycarbonate lean-to on the side of the house, and the roof has become very brittle. So much so, that it is full of cracks and small holes and one or two large ones (after the builders who just replaced our guttering dropped bits of rubble onto it  )

I would probably need it cut to size, and don't need the huge amounts that I have seen from on-line suppliers.

I ended up buying them from eBay - singly they are expensive from B&Q etc, so I bought a set of 5 for the same I  would have paid for 2.