Obtaining a Working Holiday Visa?

If a foreigner visiting Canada decides that they'd like to stay in Canada for a year and work, are they able to apply for a Working Holiday Visa from Canada while here? Or do they have to apply for it before they arrive in Canada? Normally a letter from the employee is needed before someone from overseas can take up employment in Canada. But what if someone visiting here applied for a job and got a job and the letter from their employee? Could that work?