Just picked one up about an hour ago in Austin, Texas. They had stacks of about 50 of them. This was B&M, so YMMV. It's listed here for $89.99:
http://www.bestbuy.com/site/olspage.jsp?skuId=8247266&st=guitar+hero+ii&lp=4&type=product&cp=1&id=1168045345487