in the older days of offset printing 60# offset was considered comparable (if less opaque) than the 20# bond - and in reality today you can buy 60# offset sheets that mic the same as a nice 20# bond sheet.
considering that the weight may not be super critical - other than a "
paper" instead of a "cardstock" sheet....
ENGINEERING BOND is the name of the cheap 20# bond that you can order in rolls for the wide format - Kelly
paper supplies these for us, but is is basically the regular 20# bond, white copier
paper, but in rolls. I believe every mill that makes white copier
paper also makes engineering bond - there are many brands available.
this stuff is cheap - I just looked on Kelly's catalog, 36" x 500' rolls are under $50 for a 2-pack - 3,000 square feet of
paper for about 45 bucks - can't go wrong with that.