What is the best paper (100% cotton neutral pH) for copier? Trying to make copies of old magazine ads to use in collage work.

While we don't have cotton papers specifically made for copiers, we do have many that work. You'll need to look for papers in a lighter text weight. Somerset Book is available in 115gsm and is a great choice. But look here http://www.legionpaper.com/category/2011/7/26/cotton-paper.html for a wider range of choices.