You can find it in the kernel sources:
If you did not have the kernel sources installed, grab them at http://www.kernel.org
and unpack it (the folder will be linux-x.y.z/net/wireless).
But your driver does not necessarily use it. Have a look at /usr/src/linux/drivers/net/wireless too.
If you want to recompile the wireless modules, use the kernel sources provided with openSuSE, since they will minimize the risk of breaking your system. If the sources are configured, you can simply modify the source, type make in the top-level directory and unload/load the new modules with insmod/rmmod.
I need to modify the wireless protocol stack for my project.
May I ask, why do you need to modify the wireless protocol stack? I think the linux wireless protocol stack is in a stable, mature state. If you want to send user-defined packets, you can use the packet injection features provided by some wireless drivers.