Firstly welcome order Welkommen..
Try Melvyn Rutter on the leaf springs. My thoughts are no one supplies those.https://mogparts.net/parts-guide
On the engine mounts I have no idea , but others on here will.
Why do you think poly ethylene will be better on an engine mount vs a chemical rubber like nitrile? Ploy is rock hard and surely you want the vibration absorbed by the material.That is why they are an industrial rubber compound that flexes.