This is a very common question. My answer is usually, “Nothing special, as long as it holds together when subjected to water during the marbling process.” This is quickly followed by a personal story of experimenting with a piece of thin rice paper, which acted like wet facial tissue and shredded when I tried to lift it off the marbling bath! Oops.
Many different paper types can be used in marbling. Each artist has her favorites, and the best way to know if a particular paper works well is to try it. Here are the qualities that attract my attention in choosing paper for marbling:
Flexibility: Well-suited paper for marbling must flex on-command in order to lay smoothly onto the bath. Stiffness leads to imperfections, particularly air bubbles—those round un-marbled spots on my finished paper, which elicit instant disappointment!
Durability: This refers to the overall strength of the paper. Long, beefy fibers will hold together better than short or narrow fibers. Thin paper is more likely to fall apart, also.
Absorbency: Paper must be able to readily absorb paint for proper printing. In my experience, absorbency is found in natural-fiber artist-quality art papers which have little to no sizing (light, inert starch added in the paper-making process for a smooth finish) and absolutely no coating of any kind. I've also learned that some very thin paper does not have enough bulk to be able to absorb paint fully, producing a pastel version of my vibrant designs.
My basic white go-to papers for marbling are a Japanese printmaking paper called Masa and a very special paper made only in Michigan’s Upper Peninsula called Texoprint. Texoprint is buttery smooth and easy to handle. Both of these papers flatten well after being subjected to water during the marbling process--Masa because of it's pure cellulose fiber content and Texoprint because of it's slight Latex content.
A third favorite of mine is Nepalese Lokta paper. Lokta comes in many colors and sports a delightfully uneven texture. If you’re a perfectionist, Lokta will be a challenge to print, as it scores a failing grade in the “Flexibility” test, and goes down however it pleases on the bath, producing some delightful yet imperfect results. It also does not flatten well after marbling and can take on a rigid, crinkly feel.
All of the aforementioned papers are available at the Marbling Supplies Etsy shop, and described there in depth.
Regular copy paper? Yes. Go ahead and use it for learning, as it’s inexpensive and plentiful. Unlike artist-quality papers, it is not archival, which means it contains acid and will yellow with age. Works marbled on copy paper will not last forever, and will probably not flatten well, becoming crinkly.
I particularly like marbling on US produced book pages, sheet music, and other publications from the 1880s. Occasionally it is too brittle to handle well, but paper from this era can to do very well; holding together and flattening nicely. Look for thick pages with a smooth-to-the-touch feel. They may show a slight sheen when held to the light. You'll see better results if you treat the paper with alum like any substrate used for marbling.
All of this I've learned from experimenting with many types of paper. There are no rules. If a paper intrigues you, try it! The worst that can happen is you won't like it.
..or it will fall apart and you'll think of me as you're fishing it out of the bath with a slotted spoon like I had to use with the rice paper!