I’ve ordered from an online provider in France but this company seems to have locations in the UK, Belgium, Germany and the Netherlands (or is this just a front and everything ships from France I don’t know).
Here is the UK site for instance: https://plasticsheetsshop.co.uk/
Otherwise, if you’re outside Europe, just google for “plastic cut to shape”, I’m sure you’ll find one or just find one locally in the yellow pages (if that still exist :)).