I have been in contact with the French representative of Koskisen who sent me some samples (which you will see soon in parts I will be making for the trike). Of the two places selling their ply one replied once but not to my detailed request for ply and the other never replied to my mail.
The price is one thing, the cost of shipping is another and often more expensive than the 3 sheets of 1 mm ply I need...
A family member often goes to Germany to buy second hand VW Combis I will try and negotiate space on board the next time he goes.
These sheets of plywood up to 1,5mm thick can be rolled up in a roll about 50 cm across. Such a roll can be transported in the back of a VW combi without problem. The city of Arnhem where i buy my wood at the Arnhemse Fijnhouthandel https://www.af.nl/ is near the Ruhr area in Germany. That might be near where he buys his cars, but Germany is a big Country.