The "major providers" like AT&T may try to insist on a long term contract, but there seem to be lots of smaller business, likely with their own servers, that might be willing to sell a couple of months worth.
One local Winfield KS service has been offering "duration of the festival" hookups at our big local fest (about 2 weeks for $30 US) so it can be done if you can find someone willing. It probably is easier if the service touts wireless, since there's virtually no "site prep" required - you just have to be close to an existing node.