The Americans use kraft paper, ie, made from virgin fibre, ie trees, as opposed to most of UK 'kraft' wrapping paper which is pretty near 100% recycled. They do it because they can, since trees are cheap in the US - I hasten to add that they plant and grow the trees that they use for paper-making. Having said that, they are building recycling mills for packaging papers, but because most of the stuff they recycle is made from 100% virgin, their 'recycled' paper is superb compared to ours.
I would have thought that any old thick paper would do for this kind of underlay. If you know anyone who works for a corrugated box company, they may get you a stub end of paper on the reel.
Incidentally, paper cannot be recycled indefinitely. Each time it is subjected to the paper-making process, the fibres get shorter and the paper gets weaker, and eventually you would have mush. The whole cycle depends upon the introduction of virgin fibre - usually from trees, but it could be any cellulose fibre - to maintain paper strength.
There will be a test at the end of the month, class.