By driving through Geroldsau you pass some stands where the farmers sell pumpkins, apples, potatoes and flowers. I do not know any supermarket where you could buy such huge pumpkins.
But you can only buy these things in fall - after the harvest.
What to pay: It depends on how much you buy and how hungry you are. You can get a small pumpkin already for 2-3 EUR
Especially in a hot summer it is nice to walk to the waterfalls sinces it is not so hot in the forest.
It is a nice walk along the creek. Near to the waterfalls is also a restaurant called Buetthof.
For more pictures please the my travellogues Geroldsauer Wasserfaelle