I understand from the ski and snowboard book, that we can buy discounted ‘overseas traveller’ lift passes from travel agents without having booked the whole holiday with them.
 
I have contacted several agents without any luck. 
 
Do you have any suggestions as to which agents we should contact, or if there is any other way of getting them?
 
I would appreciate any help!

One resource you could try is the online agent liftopia.com
 
They are not a travel agent, but they specialise in offers on US lift passes.