Hello everybody:
Unfortunately, nowadays it's not as easy as show up in Cusco and shop around for a tour to the Inca trail, specially on the high season (June to Sept.), since year 2001, new regulations have taken place and it's harder and harder to book it. This year new rules took place, first of all, the limit today is 500 people per day, including guides and porters. Also, the tour outfitters have to confirm and pre-pay for the spots 45 days in advance.
It's likely that you will be able to find last minute spots, but it will be expensive, and you will have to spend more time than what you planned just waiting to start the inca trail.
I highly recommend to book it in advance.
A big Peruvian hug