It was in one of the Zahn books, the one when Mara and Luke go out to find the remnants of the ship the original Jorus C'Boath and fleet went out on. I can't recall the title or anything but I recall that particular fact was mentioned it wasn't until right before the forming of the Galactic Empire that they were founded.
Still, 3000 years is a long time. If the Jedi can forget Kamino exists, why not the CHiss.