From Amazon:
36x36x0.062 is $22.50 - virtually the same size as the K-GAP
48x36x0.125 is $49.00
33-005-062-036-036 General Purpose Rubber, 50A Durometer, Smooth Finish, No Backing, 0.062" Thickness, 36" Width, 36" Length, Black
https://www.amazon.com/33-005-062-036-036-General-Purpose-Durometer-Thickness/dp/B00P5V72OQ/ref=pd_sbs_328_1?_encoding=UTF8&pd_rd_i=B00P5V72OQ&pd_rd_r=7f194559-b87f-11e8-85dd-873fcc9bd247&pd_rd_w=nJU5c&pd_rd_wg=hN7vX&pf_rd_i=desktop-dp-sims&pf_rd_m=ATVPDKIKX0DER&pf_rd_p=0bb14103-7f67-4c21-9b0b-31f42dc047e7&pf_rd_r=3H0YFGPJAGF8BJYET73B&pf_rd_s=desktop-dp-sims&pf_rd_t=40701&psc=1&refRID=3H0YFGPJAGF8BJYET73B
33-005-125-036-048 General Purpose Rubber, 50A Durometer, Smooth Finish, No Backing, 0.125" Thickness, 36" Width, 48" Length, Black
https://www.amazon.com/gp/product/B00P5V9186/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1