It's a random thing. When you entering the lake, you have a chance to pick the shroom. Keep trying, get 1, plant it at home, get 10 - plenty :D
A i see, so its not in the lake ,but when i enter the lake room that i have a chance to find one, also thanks for the planting tip. i will try that.
Then my ridiculous luck is broken, i found 2 the first time thanks to the big perception levels i had and then got 20 after planting