Rugby architectural building supplies. I get baltic for about $1.20 a GF-size piece (it’s like 15$ for a 5x5’ sheet) The 1/8 MDF is even cheaper, like 7 or 8$ for a 4x8’ piece.
As for materials, that’s just ply, you should see my hardwood stash 
It’s a sickness, man. I gotta stop buying hardwoods.