I got this one (2' x 4') for 31 bucks off amazon.
https://www.amazon.com/gp/product/B007J21L0A/ref=ppx\_od\_dt\_b\_asin\_title\_s00?ie=UTF8&th=1
Amazon, $29.99 for a 2'x4' piece.
Fastcap 1-1/8 Thick Kaizen Foam, 2' x 4' https://www.amazon.com/dp/B007J21L0A/ref=cm_sw_r_apan_glt_i_ARBM89VNWB51XX42QWWP