You'd need to get sponsored for a visa to work in the U.S. (or at least I do and I'm from the U.K.). I got what is called a H2B visa which is for seasonal workers, they are not that hard to get you just need to find a job first. You could apply as pretty much anything, but most resorts seem to sponsor a lot of instructors and a few lifties as well.
Good mountains to work for depend on your job, but I'd go with a resort in the west - Colorado (such as Aspen, Vail, Copper, Breck, Winter Park) , Utah (Park City, Alta, Snowbird) or California (Heavenly, Squaw, Mammoth). Check out the resort websites for available jobs.
It's easier to get a visa for Canada as you can a working tourist visa without already having a job, then just turn up and look for jobs.