I've run my own tours, so far just with photo friends. Normally using Bombardier coaches rented from Xanterra.
We generally stay at the Old Faithful Snow Lodge within the park.
Most of the canned photo tours run out of West Yellowstone.
A web search should point you in the direction of these operators.
David Brookover and I are considering running some open tours in 2012.
The problem is, when you run tours on a commercial basis, I for one feel you
have to almost guarantee success. Wildlife types want wolves, landscape types want a winter wonderland.
The park will always give you something great, but what it is varies.
Last year a lot of people spent winter weeks in the Lamar Valley with nary a wolf closer than 500 yards.
We however, lucked out along the Madison River, go figure.