Portobello Mushrooms are one of the few different Mushrooms you can find in the game. As they are in real life they're more expensive than just the regular mushroom and a lot more rare than the regular type of mushroom. While not impossible to obtain you'll get them about 1/3 the amount of time until you get into 60 - 70 zones where you'll find Portobello Mushroom nodes all the time.
Northern Mount Maelstrom in Gauntlet Gulch by the Gauntlet Waypoint.
This is a great location that you can return to every hour and a half to gather up all of the Portobello Mushrooms again. Sometimes they spawn sooner, I am not 100% sure of their spawn times.
Below is a picture of how many Portobello Mushroom spawn locations are here.