In Big Timber you can fish both the Boulder and West Boulder. There are several fishermans access points on the Boulder. To fish the west you need to go about 25 miles to the only access point. Both roads go into box canyon,the area is just beautiful. There is hiking and plenty of fishing. There is a very nice CG just south of Big Timber right on the Boulder. There is a fly shop on the main drag that can be very helpful.
While there go to Pioneer Meats, they have some awesome bacon and fresh cut meat.
On the West Tom Brokaw and Michael Keaton own the first ranch. You can access their property and others at the bridges and you can fish as long as you don't go above the high water mark.