You can catch PARAS and half of the time they should have a tiny mushroom it's evolved form has big mushroom. You can also teach one of your pokemon Thief or Snatch and it saves time and poke balls.
you can get mushrooms if you go to mt moon and use the item finder in the lower levels near rocks
Catch a bunch of Paras. They always hold tiny mushrooms. And Parasect also holds a tiny mushroom.
go to Pokemon tower on the 6th floor use the itemfinder
Nope as long as you have big mushrooms or tiny mushrooms you can have the move tutor teach Pokemon moves forever.
No, but they do carry them. If you want lots, catch as many of them as possbile, then release them.
No, you cannot find cyndaquil in LeafGreen.
A mere Cameo appearance from Firered and Leafgreen. They do no good, so you may as well sell them.
I found one in the last room of Mt. Moon hidden in the furthest top right rock.
LeafGreen version does not have Growlithe.
There aren't growlithe in LeafGreen. FireRed has growlithe, LeafGreen has vulpix.
They can be given to the Move Relearner to teach a Pokemon a Move it's already learned in FireRed and LeafGreen, but serves no purpose other than to be sold in Diamond and Pearl.
That item doesn't exist in leafgreen.