Negapon
Once you unlock Marvel Lab, enter it. Make sure you have room for at least one more Robopon. You should see a building directly in front of you. Go to the left of that building. Once there you should see a dog. Talk to it. It will tell you to input a password. Enter '9'. The dog should give you Negapon. Negapon is very powerful. It turns into NegponX and Gatspon.
Unlimited Mushrooms
When you find a Mushroom of any color, get it and store it in your warehouse. Then, leave the room/area that you are in. When you return, the Mushroom will have respawned. As long as you keep storing them in your warehouse, you can get an unlimited amount of Mushrooms.