I've had luck above the niobrara entrance into the Missouri. Specifically below the sunshine bottoms in the cedar forested area on the Nebraska side. Some ranchers run cattle in there, so you may have some bovine company from time to time. Also just up river from the verdel landing and the blue moon, after the cabins end, there is a nice stretch of cottonwoods that have always yielded in the past. Down near L&C, in the area around Weigand campgrounds to the west have been good too. Start in the open grass areas close to the tree lines
Good luck & happy hunting!
Shroomani