There are outfitters who guide in Yellowstone, but really the place is largely self-guiding if you're sticking to the roads. In two days you could see most of what there is to see, especially if you plan ahead a little. This is a decent website for just that; there are maps you can download and mucho info:
Yellowstone National Park Vacation | UltimateYellowstonePark.com: For Yellowstone Travel, Vacation, Hotel and Real Estate Information
Everything in the park is clearly marked, and the biggest obstacle to your planning will be the traffic jams that occur when people spot wildlife near the road and plug up the whole works for anywhere from a few minutes to an hour or more.
The best way to avoid a lot of this is not going to sound like ideal vacation behavior, but getting a pre-dawn start to get in to where you want to be helps greatly - traffic is thin and you can avoid all the bottlenecks at the gates. And seriously, very early morning is the best time to see things that you might not necessarily spot during the day - late evening can be the same way. Early is best because to me it's the prettiest time of the day, and all the critters are out foraging after the night's over. Crowds are minimal and it's much quieter.
Spend the busiest roadway times of the day at a destination, i.e. Old Faithful/thermal areas, Lake, etc. You'll have some crowds there but won't be stuck on a highway somewhere just waiting for traffic to clear.
If the self-guided thing doesn't appeal to you, google 'Yellowstone tours' and it will list quite a number of businesses that offer a wide variety of options. If you're in Big Sky, the Conoco station on the corner of the spur road to the ski area has a whole rack of brochures, and so does the chamber of commerce.
Hope this helps, and have a great trip!
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Yellowstone Visit
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