Don't think rubber bands. You need Tan Super Sport from www.faimodelsupply.com
. It comes in long continuous strips, you cut off the length you need to be 2.0 gm and tie the ends in a knot to make a loop.
SOME hobby shops carry Tan Super Sport too, some other online sources, but FAI Model Supply is the original source. I'd suggest you start with a qtr pound of 3/32 wide.
For kits, unfortunately no currently available kits meet the rules. The existing Wright Bat kits have the propellor already made, you have to make your own for the event. The other common kit is the Penni Copter, I think its rotor is also pre-made.
Better to look back up this chain for hints and tips on designs and build from scratch. There's a Wright Bat design on the Illinois site, but to be legal for nationals you need to change the rotor from the plastic cut out pieces to balsa. There's some nice twin rotor designs that are WAY to hard core for these rules (less than 1 gm copters) but the design approach will work. There's also a video of a good design concept on youtube pointed at a way back up the chain. Flights are kind of short, but the basic design is capable of more.
Its probably better to build from scratch than to wait on a kit to arrive that you have to modify the heck out of.