If you use cork in your yards (which isn't really necessary IMO), you'll have to elevate the mains that much more if you want realistic separations between sidings, A/D tracks, and the yard ladders. The mains should be higher by about 12" or so. Again, not strictly necessary, not on a layout, but the real roads would definitely have those lesser tracks lowered to keep loose rolling stock from running out onto the mains.
Most, many, hardware stores or builders' stores will have rolls of cork for underlayment. Then, with the proper amount in hand, you use a utility knife, straightedge (a 4' metal level or framing square is great), and some clean/clear space on a floor, you cut strips about 1.1" wide.
You want the stuff that's about 5/16" thick...ish.