The note can be found during the Brotherhood Caravan random encounter.
ORDERS: Bring collected supplies to Thunder Mountain Power Plant
It is imperative that you gather supplies from the citizens by any means necessary, short of using force. Without the food, medical supplies, materials, etc. we cannot defend this area from the growing threats. We protect them and they in turn support us.
The citizens of Appalachia are becoming restless and doubt our effectiveness, but you will remind them of the latest attacks that we were not there to intercept. If further convincing is needed then tell them that they can deal with the next Scorched horde on their own and see how they fair.
We are depending on you.