If you need several sheets, 20 or more, and have a truck, and a cover if it rains, there is a hardware/lumberyard that stocks this in Nashville, Tennessee. Come to Clarksville and I will go with you, I used 28 sheets on my layout, it’s a great product for sound deadening, and screwing down track. It is quite dusty, so cut it outside. Celotex is a mess, will not hold screws well, is crushable. There are lots of considerations, thinking processes, before you actually buy this product. If your using 3/4 plywood, built on 2 by 4 framing, you really may not need it. Old toy trains, rolling on sectional tubular track on 1/2 inch plywood made vibration noises. If your using Gargraves flexable track, Atlas O trackage, or the new fast-track and you use carpeting pieces under your table legs, that will really help reduce train noise.
Homesote is a great sound reducer, however, you would need to paint it to seal it before laying track. By painting the Homesote, you can use 1/4 inch cork roadbed, then screw the track directly into the Homesote itself. Not into the plywood, thus eliminating vibration sounds.
Good Luck, Happy Railroading.
