plexiglass or similar (Higherford BB9)

I need to cover my bedroom windows for winter, and can't use screws or adhesives, don't ask !!!! Looking for plexiglass, 2mm thick or more, clear, minimum size per sheet 1m x 50 cms, and need 2. The sort of plastic you find in large poster/picture frames, basically.
I can cut it to size using a craft knife, so doesn't matter if the sheets are bigger.
Probably wasting my time asking as the stuff has become seriously expensive lately, but if you have any I would be pathetically grateful.
Don't drive, but walking/bus not a problem.
Thanks for reading.