I know of no guides but I too am taking the trip with my Fraulein in the same month. Her brother is taking up a job in the park for the summer so we are going to visit him. Let me know if you come across any good web sites that you think are above average for Intel. I will keep my eyes open and ear to the ground for any guides that come recommended.
Most rivers in the park open on the last Saturday in May, though only a few will be fishable. The Yellowstone River doesn't open until July 15 to protect spawners. Yellowstone Lake opens on June 15th. For more on the regs, http://www.nps.gov/yell/planyourvisit/fishing.htm.
As mentioned earlier Blue Ribbon Flies is a great outfit.
Also check out www.flyfishyellowstone.blogspot. com.
Enjoy your trip.
Just to clear something up, the rivers are open the last saturday of May not the 15th of July. Thats the general opener but there are some rivers/tribs that do open later but for the most part May is the opener. If you are in the West part, Blue Ribbon is your best bet. They are good people and great guides. Around gardiner there arent as many guides but really, Yellowstone is the best place for DIY fishing and June is the best month cause its before the tourist hatch. Goog luck and if you need any more ideas you can pm me, i spent a lot of time inside the park over the last six years
Sorry for my bad info. I didn't look it up. Just going on my experience in June a few years ago, where the water I was interested in wasn't open, and was entering the height of spring runoff. I ended up fishing on some little brook trout lake.
You could easily fish the Firehole without a guide. Where it meanders through the midway geyser basin it's an easy river to wade or cast from shore and a short hike will get you away from crowds. It's small water and full of willing rainbows and browns, and you can keep a few to fry up at camp after a full day on the river.
Vojak: Go to www.yellowstone-natl-park.com/fish.htm for a good summary of regulations in the park. Within an hour of West Yellowstone are some of the great streams of America. Across the divide in Idaho, about a half-hour drive is the Henry's Fork. All of the West Yellowstone guides know and fish these streams.