Being that there were three/four different variants of the G4 15", which do you have? They may have different panels internal, depending on model. Once you know which one you have, finding a similar panel equivalent on
Panelook.com might help in your search. I am going to assume it is some LG/Philips panel.
It looks like UsedMac.com says they are all the same (
link to panel).
Here is
one on eBay.uk
Here are some
on PowerBook Medic.
In Europe,
this link to TheBookYard.com looks to be out of stock.
DV Warehouse is also out of stock.
And some reading to do on
another forum.