After spending a lot of time with the presumably correct kernel source for my RPi in order to compile a driver (kernel sources for the driver module and the built kernel need to match precisely, settings included) I gave up and decided to start from scratch, so I downloaded the latest Raspbian image that has kernel 3.18.7+. If I understand that correctly, the + means that there are additional commits on the top of the 3.18.7 kernel release, so I can't simply download those sources, because the driver would fail.
How can I download the correct kernel sources for this build?