You don't need to order cheap materials online; your cheapest materials are available in your recycling bin and your hardware store (or in my case, my garage and my basement paint closet.)
The best way to construct a float is with papier-mache, so all you need is chicken wire, Elmer's glue, old newspaper, and latex paint. (And maybe some lumber scraps and a staple gun for framing if you are up for trying sculptural elements.) Add glitter if you want things to be shiny.