@Hdip this is 2 years old but I use Foam Sales Group which you can google easily. (They are moving so I’m not posting links.)
Guidelines for people trying to buy “the right foam.”
EPS Expanded PolyStyrene. I use 1.5 lb (pound) density. Some people use 1lb and some have used 2lb but that’s really too heavy.
Normally you can buy a 4’X8’ sheet, 4" thick, in 2021 for between $50-$75. So while you may worry about buying a big piece, if you can pick it up, you can get a lot of foam cheap. I recently bought a 5" thick 4X8 sheet for under $60 US.
If you’re not in LA, just google industrial or construction foam suppliers and call them. If they don’t have EPS, they will know where to get it. It’s locally available in most places since it’s used in many industries. It seems to ship in blocks that are 1-2 feet thick. Usually you can get the thickness you want cut.
Hope this helps!