If you want a full 4 piece bodykit, you are going to have to import it from Japan. Those kits will set you back between $2-3 grand - this price is excluding taxes,shipping and paint.
The only companies in the US that make anything for the G20 are below.
www.wingswest.com makes a front lip for the G20 and
www.stillen.com makes a front bumper cover.
Nismo is coming to the States soon, or should be in the States - so you might want to wait and see if the Nismo kit will be cheaper, than if you were to import it from Japan.