Original Source: Play Create Explore
I used square baking tins (Dollar Store people.... 4 tins=$4) and they worked great. I've seen people use boxes as well. My kids would use the paint squirt bottles, cut a piece of paper to size, drop a marble in (or 5 marbles), and go to town. Great for a center!
These marbles can be found at Walmart, Target, Hobby Lobby, etc. (the ones people put in glass vases).
I love these. Aren't they gorgeous? Perfect for framing in the house too. A couple mom's told me they did it at home using colors that went with decor. Smart.
Marble art in my room: