You would have to download them from somewhere but only IF they are available....and this is quite rare. Garmin sells a $100 US TOPO CD that has all the forest roads, lakes, powerlines, pipelines, elevation contour lines, marshes, railroad grades, etc...but they don't specifically have the snowmobile trails identified on them unless they happen to follow those same pre-existing features. They look alot like the USFS topo maps if you've ever used those before.
It's largely a hobbiest thing and very spotty as far as the snowmobile trails are concerned. There's a few clubs and individuals that are very technically savvy that offer downloadable trail maps for their own little local area, but they're few and far between. Probably will be a few years before they are readily available and better quality.
Hopefully downloadable snowmobile trail maps become more common and easy to find over the next few years, then it will be a more useful on-trail tool.