I've got a wild cherry tree in my back yard which has cherries around the first week of June every year. We've made pies and such in the past, but this year the harvest came about a week early due to all the heat and early spring rain. I've just finish picking the 5 pounds I need for this batch over the course of the last few days and am looking forward to brewing Papizan's Cherry Fever Stout soon! (cherries are in the freezer for storage)
BTW, when you're hand picking cherries off a tree, you'll see what justifies the $7/# price at Krogers that someone previously mentioned.
The official recipe also says you can substitute fresh raspberries for the cherries. It actually says to use 3# of sour cherries and 2# of choke cherries, or 5# of sour if the choke are not available. I'm using wild cherries because they're growing in my back yard.