You can buy some at the Marketplace... If there is a stock, which was not the case for me

In Khuruto Cave, go until you reach water, then go right. They are little red spots on the ground.
You can activate your pet skill "Special: Finds Resources".
You can also switch servers, now that there is not cool down between switches.
It took me 30 mn ans 846 mushrooms before getting some hypha. Good luck.