stripping them from dead units is your best bet.
it might be easier to find them if you open your search up for dead back PCBs, i'm sure people have them around and could help you out.
although, that part is something i'm working on having manufactured. it'll be several months though, so doesn't help now.
alternatively, you can use a 6-pin firewire port, i've done this before. there is a kobiconn one whose footprint is almost the same, if you'd like a part # i can track it down. you'd just need to change the ports on your gameboys to the firewire type, and use them on your project as well. then you can use the standard 6-pin firewire cables. worked great for me doing a project probably similar to yours...
I might have a firewire port hanging around somewhere (dunno if it fits yet), but it would be nice to get that part number.
I have stripped some from dead units, but I still need more.