A good way not to miss any shroom is to collect them into the inventory with pouch and then eat them all at once. The best place to find mushrooms is north-west corner of the map (Big lake in the snow biome and sorrounding area) - you'll find 5 of 6 mushrooms there, the last is the red one, unfortunatly i can't remember where they grow, but i think you can find them in the woods.