About 20 minutes North of Cranbrook the Kimberley Riverside Campground is in a very nice setting on the St Mary's River.
The St Mary's River is highly rated for fly fishing. I don't fly fish so I can't tell you how it is right around there. Further east the Elk River is highly rated but I can't comment on the campgrounds.
The Elk might have better access.
For camping in the area around Cranbrook, or Fernie, you can choose from several provincial campsites. Try this website to see which ones suit your needs.
At the end of July, most of these will be quite busy, and reservations may be necessary. Kikomun Creek on Lake Kookanusa and Mt. Fernie just outside Fernie are probably your best choices. There are others in the Cranbrook area. If you want private campsites, there is a nice one at Ft. Steele, about 10 miles east of Cranbrook. There website is
Also Sparwood, 10 miles east of Fernie has a municipal campsite. It is called Mtn. Shadows.
For fly fishing, you will cross the Elk River a few miles north of the Rooseville border crossing, which is north of Kalispell. If you turn east at Highway 3 towards Fernie, you will shortly see the Elk River on your right parallelling the highway right through Fernie to Sparwood. Most of it is easily accessible. There is at least one fly shop in Fernie that can line you up with guides, and Elk River floats if that is what you wish.
Sparwood and Elkford are housing communities for coal miners working in the area. For the most part the mines themselves are out of sight, or visible as flat top hills in the distance. The Elk and Fording rivers are in good shape.
Several years ago I drove to Elkford and did some hiking. I was thinking of driving further up valley to an alpine prov. park, but decided the road was too potholed, and returned to Sparwood to camp. There is also a municipal campground in Elkford.
I suspect that most of the campers and fishermen in the area, other than locals, are from Calgary. I also encountered a lot of motorcycles returning to Calgary from a weekend spent in the Kalispel area.