To get to Barrow Fields, you go thru Great Wood and into Darkwood.. After Darkwood you'll find the town. There's a guy there who will sell you the last blue mushroom. It costed me 1500. I thought maybe I could buy 2 from him (since I need a 4th now) but it's the last one. For the one in the woods, you gotta go to the Hero's Guild and talk to the guy in the woods. He will give you a letter to give to a girl in Oakvale (which is a couple towns from Barrow Fields). In return, she will give you a letter to give to him. He will then give you a blue mushroom.
If you have already given mushrooms to the witch, then you may wind up with the same problem I have. Let me know how that works out =0)