It' really hard to find shrooms where I'm from. I found a 8th but the person said they have been sitting in a bag for a month. Does that make them less potent?
They grow wild in every state, just different species...(check out the Erowid site.) I think that's where I saw them. Trouble is, you have to be very careful with what you are finding. You don't want poisonous mushrooms instead. Cow pastures are a good place to start.
If you give them long enough the oxygen will destroy them. Short term in a bag not so much. But your dealer says one week in a bag that he knows of. What about his dealer? Only way to know is to eat them I suppose.