nutz.Send SASE's everywhere and anywhere. Ski companies are usually pretty down to send people free stickers as it's basically cheap advertising for them.
This is basically it. I sent a bunch of letters a before the start of this season, they started rolling in right about at the start of the season. Most companies are fine with it, some even send nice die-cuts.
Try your local ski shop, normally they have boxes full. Otherwise, what the other guy said, send Self Adressed Envelopes to Ski brands asking for stickers. I personallly know that you can e-mail Monster to ask for stickers and they will send you some.