Race rules are constantly changing and evolving. Some races allow personal or outside GPS trackers, others don't. And this is regardless of whether or not the race organization provides their own during the race, which they aren't always able to do. Here are a few resources you can check if you're trying to get up-to-the-minute GPS tracking on a particular race.
Iditarod: subscribe to an Iditarod Insider service featuring GPS.
Many mid-distance races also include GPS tracking, often for free. Some to check out:
Copper Basin 300 GPS Tracking
Knik 100 & 200 Tracking
Kuskokwim 300 GPS Tracking
The Goose Bay (Tug) 150 & Tustumena 200 are often available on TrackLeaders, as are numerous other mid-distances races.
Check the TrackLeaders website or the respective race site closer to launch time.
Willow 300 GPS Tracking
Denali Doubles is also planning to work with TrackLeaders. Check here for updates.