I use Marble Thix with my students. It is simple to prepare (just
sprinkle some on top of the water the night before or use a mixer/blender
if under time crunches). While the small bottle of powder may seem
initially costly, you don't need much for an activity. I tend to use the
same mixture for at least two classes.
Another very simple (and cheap) way of paper marbling is to place a piece
of screen over a container of water. Children enjoy running chalk (great
time to use up those scrap pieces of chalk) over the screen and watching
the chalk powder sit on the water surface. When a piece of paper is
placed on top of the water and then lifted up, the kids are always amazed
at the results.
If you are really interested, you might want to check out the book
"Marbling: A complete guide to creating beautiful patterned papers and
fabrics" by Diane Vogel Maurer w/ Paul Maurer, Crescent publications
ISBN # 0-517-02019-X.
Hope this helps.