An electric carving knife works great on foam
Yes approach store management and explain need and amount they should work with you
About 1 inch
Fiskars model 9421 (03-020267) Kneeling Cushion bearing UPC 0-46561-29421-2 is 11 x 18 and I bought it at Home Depot for $6. I was looking for one 18" square or larger, so I bought two.
10-7/8" x 18"
Its 3/4" thick. I am 275lbs and it compresses under my weight but bounces right back once I get off of it.