Dead easy. Either call in at the Markony Bar (at the enterance to the slopes,) many of the instructors who do not have a class will be in there, or just ask at the ski depot and they will arrange it for you.
Negotiate..... whatever they ask you. But for 5, I would suggest a hundred quid for the week sounds a good starting point and see what they say. You are better off speaking to an instructor direct as that way he/she does not have to pay a commission to the ski school.