perspex (Kings Meaburn CA10)

Hi - I'm looking for a couple of pieces of clear plastic glazing material (Perspex or similar) for the top of a small greenhouse. Each piece needs to be 3mm thick and at least 515mm by 290mm. I can cut down anything that's a bit bigger. It doesnt need to be pristine - a few scratches etc wont make much difference. Thanks in advance, Gerry