The solution on which the colors are floated is referred to as the size.  The size can be made of several different types of gel or gum, but the finest and most detailed marbling must be done oncarrageenan size.  Carrageenan is an extract from a seaweed called Irish moss.  Some marblers still use the actual seaweed to make their size, and it can occasionally be found for sale, but it is a real hassle compared to its commercial extract, carrageenan... the Irish moss has to be chopped, dried, boiled, cooled, strained, and filtered, and the resulting size is brown instead of transparent, which makes it hard to see your colors.  

We sell carrageenan in half-pound and one-pound bags.  The half-pound bag contains enough to produce over 300 papers of the 9½"x12½" size, and the large bag will make over 600 sheets (as long as you don't throw out your size as soon as it starts to get dirty with excess paint... you can do perfectly fine marbling on dirty size.)  So a bag of carrageenan will go a long way.  

Pronounce it care-a-GHEE-nan, not care-a-JEE-nan -- the "g" is pronounced as in "get", not as in "gel".  It is a food product, used as a thickener in many products such as ice cream, chocolate milk, yogurt, cottage cheese, etc -- you have been eating it all your life probably without knowing it.  It is a type of gelatin, though it remains liquid instead of setting up solid like Jell-O.  You must prepare it in an ordinary kitchen blender.  It will not harm you or the blender you mix it in.   This is how to make a half gallon of size:

            Put 1 level tablespoon plus 1 level teaspoon of carrageenan powder into an empty blender cup.
            Fill it up to the top with cold water.  
            Blend it on the highest speed for one minute. 
            Then pour this foamy mixture into a half-gallon container (such as an empty plastic milk jug) 
                            and fill it on up to the top with water.  
            Shake it a bit to mix it.

Let this mixture stand overnight in the refrigerator, or for at least 4 hours.  This is to let the bubbles pop; it comes out of the blender as a mass of foam, and has to sit many hours before it all becomes liquid again.

To make a whole gallon of size, use the above method but put 2 level tablespoons plus 2 level teaspoons of carrageenan powder into the blender cup, instead of 1 of each.  Blend it one minute as before.  The resulting mixture will be much thicker than the above recipe.  Pour it into a one-gallon container and fill it on up to the top with water.  Shake, and let stand several hours as before.

The carrageenan mixture will last about 10 days in the refrigerator, but only 1 or 2 days at room temperature.  After that, it begins to lose its thickness and becomes watery, and is no longer good for marbling.  Only a regular kitchen blender should be used — mixers or food processors don't go fast enough.

Ordinary tap water is usually fine for making the size, but occasionally there are problems.  If your water is heavily mineralized or hard, that may prevent the carrageenan from gelling, and so all your colors will sink instead of floating properly.  Water in desert cities such as Las Vegas or Phoenix sometimes is not good enough for making the size; neither is water on some sea islands.  Often country well water also will not work well, if it is hard or contains lots of minerals.