We booked through Scenic Safari to take the tour up to Old Faithful at end of March 2014.
They have everything to outfit you properly suit/boots/gloves...if i were to do this again I'd go lighter on my own personal clothing... you'll thank me later... if not only for the hour+ ride to/from the park in the van with 10 other people and all their gear.
Do bring a scarf though as there is that bit between the suit and the helmet the wind slices right through. Maybe glove liners too...
Note: doing this does make for a long day (80+ miles on the snowmobile). So if you don't love snow / being out in the elements, this might not be for you. Seeing Old Faithful actually ended lower on my list of this tour's highlights but really though...it's beautiful out there. I'm so glad I went.
A few side note(s)...definitely eat at LOCAL. One of the top meals we've ever had. Other meals we enjoyed: Cafe Genevieve, Bunnery Bakery, Silver Dollar Bar and Snake River Grill. The Couloir Bar was vastly disappointing, there were 6 seats at their tiny bar. (Can't speak for the view as it was heavily snowing and just saw a wall of white). Nora's Fish Creek Inn and The Rose "Speak Easy" Bar weren't memorable for us.
Besides that, the stay at the Wort Hotel is highly recommended! And half day snowboarding lessons at JHMResort...you'll be aching the next couple days but it's worth it for the experience. Say hi to Ryan!!