First the Mask of Scents can easily be found by following the buttler in the Deku Shrine, which is to the left along the lilly pads as you enter the Deku Palace in the Southern Swamp. Remember, you don't have to race the dude just follow him till you get to the end. second there are 4 places I know to get mushrooms. Obviously one of them I can't get to. 1. In East Clock Town in the Stock Pot Inn, inside the third room on the second floor in uhh... Gorman's underwear.
2. This one you can't get to but it's in Stock Pot Inn in the toilet (I believe that's what it is) on the bottom floor first room to your left as your going down the hallway.
3. By the owl statue in southern swamp.
4. Somwere in the Woods of Mystery. Use the Mask of Scents and an empty bottle to get a mushroom.