The local foam mfr for me, Imperial Foam on Rt 40 outside Ormond Beach. FL will sell you whatever quantity you need at a reasonable price. Not just sheets, but also CNC wire cut shapes. I've used them for other non-aircraft shapes, but if I gave root & tip airfoils on CAD, plus a wing layout, they'd even cut wings for me.

This from a place that expands it's own styrene beads and cuts the blocks into slabs for industial use and vairied shapes like fake columns for architectural decoration.