there isn't really a specified place to find the .config of your last
kernel build. usually, when you finish configuring and save the config, it
will be present in the kernel src dir.
i don't have any .config under /boot cos i build them in my /home and save
em under one folder.
so, if you did a kernel build a while ago, look in that src tree and you
should find it there.
if you havent, then i think you will find it under /boot (you could just
do a 'locate .config' and that should tell you if you dont find it there.)