I think the most common sources are:
* Plastic netting * Plastics from countries without any/lacking waste infrastructure
There are differences in statistics between EU countries, they might tell something interesting.
EDIT: "considered to be at risk of missing one or more material-specific targets, with plastic being the most critical material:
Plastics: Austria, Bulgaria, Croatia, Cyprus, Denmark, Finland, France, Greece, Hungary, Ireland, Italy, Latvia, Luxembourg, Malta, Poland, Portugal, Romania, Slovakia and Spain" https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=COM%3A20...
Heck I wouldn't trust paper food packaging because I'm not sure they would use a wax liner instead of some fancy PFAS forever chemical.