You can get composite panels, extrusions, adhesive etc. from Ormocar https://www.ormocar.de/, Nomad Campers https://www.nomadcampers.de/ , Paneeltec https://www.paneeltec.de/, Moser Fahrzeugbau https://www.moser-fahrzeugbau.de/ , just to name a few of the more reputable ones in Germany.