It's the end of the season and someone probably has some comps floating around unused. Take a walk through the rental shop, ticket windows, etc and see if anyone wants to trade something for their comps.
I just got back from Jackson on Sunday. I don't think you can get them anywhere except on the mountain. Ask around for comps and people who may have multi-day tickets they can't use. I paid a guy in the Mangy Moose $20 for a 5-of-7 for my final day of skiing; he was leaving early and couldn't use the last day. Good luck.