Depending on how large your big top is, and whether or not its outdoors, fabric can be prohibitively expensive. In the past I have used rolls of poly vinyl for indoor applications... it is colorful, water resistant, flame retardant, much lighter and cheaper than other options.
You can probably find it cheaper, but heres a good example:
http://www.stumpsparty.com/party/Red...-Retardant.cfm




