Quite a few options for late November/early December, but not through an agent, do it yourself is best.
We ski early December most years at Val/Tignes or Val Thorens.
Several hotels do very good deals before the 18th of Dec, (aprox half price rates) they often include extras.
For example this December We are staying in Tignes le Lac at a 3 star hotel (ski in/out) for 4 nights halfboard (breakfast, afternoon tea and four course evening meal) 3 day espace killy pass for 340 euros per person!
We booked £50 flights with BA from Heathrow (ski carridge free as part of your luggage allowance), hire car from Geneva from holidayautos £160 for 4 days job done!!!
Same holiday after the 18th will triple in price!
Snow at these altitudes is usually good even in mid november, Val/tignes open 27th Nov, Val Thorens opened yesterday 20th november.
Hope this helps