The Yellowstone is a float river downstream of the park, but we typically run most of our hike-in walk trips in the canyons inside the park. There's plenty of places to not see any other anglers if you can shed some boot leather...

I'd be pleased to help the OP's friends out, though PFS certainly isn't fancy. I've taken to letting other folks and my websites do my recommending for me, rather than tooting my own horn as much as I used to. www.parksflyshop.com and www.ycflyfishing.com.