Almost everywhere except the Firehole River would be a good choice. For some like the Madison and Gibbon rivers early and late in the day offers the best times. Terrestrials insect are now a major food item for trout everywhere. Concentrate on banks and overhangs not in direct sunlight On still waters, such as Beula Lake shown below, speckled dun emergences are bringing gulpers to the surface. Adult damselfly patterns will also bring top water responses.
Many small stream are great places to try now. They remain relatively cool because of the high elevation. Some of the more convenient, because of roads nearby include the upper Gallatin River, Grayling Creek, Soda Butte Creek, Gardner River, Lava Creek, and Aster Creek.