So, one bit which could make finding the source code slightly confusing is that your grub is of the original instance, now known as "GRUB legacy".
Current development, and easily accessible source code, is all for what used to be known as GRUB2, and is now simply GRUB (with a version number >= 2). Development versions of GRUB2 were known as GRUB 1.9*.
So, I don't know if this is helpful at all, but to access old versions of GRUB sources, you can either look within your distribution packaging system, or in ftp://alpha.gnu.org/gnu/grub/.