I received a verbal job offer today and accepted, cancelling 2 interviews. There is no letter of the job offer. I start tomorrow. Is a verbal job offer acceptable or should there be a letter?
As has been indicated in comments, there's simply not enough time between acceptance and the job starting for you to receive a formal job offer in the mail and return it before you start.
Simply follow the instructions regarding starting work and go through the paperwork on arrival.
If you don't like what you hear when you're there, then head right out of the door again (in a polite fashion, of course).