This street map is very detailed, though it doesn't encompass all of Park Slope.
The handy Brooklyn Streetwise maps are easy to use and carry. Buy it!
Here you can select a subway map of Park Slope from the drop down neighborhood menu.
The entire subway map might also be helpful to give you a sense of where Park Slope is in relation to the other boroughs.
It's easy to get lost in Prospect Park. View a map of the park.