RCFoam.com 1.3# EPP foam sheets from 3mm up to 4" thick in 36" length sizes and various widths, 1.9# EPP foam from 3mm up to 9mm also 36" lengths and various widths. They also carry EPS foam, Depron foam, carbon fiber and have a variety of rc parts as well.
FlyingFoam.com Carries 24"x36" sheets... 3mm, 4mm & 6mmsheets with 2" and 2 1/2" blocks also available.
RCSkyFlyer.com Carries 12"x12", 12"x36" and 24"36" Sheets in both 1.3# and 1.9# densities... 5mm, 6/7mm, 9mm, 15mm, 30mm and 60mm thickness's. Also carries some carbon fiber of various types ie strips/flats, rods and tubes.
EppFoam.com Various widths/lengths of block EPP foam 2" thick. They will custom cut any size according to their website.
RCPlaneBuilder.com They carry 1.3# density in 3mm, 6mm and 9mm thickness's... 24"x36" sizes.