Answer to "How to fix the fading blue light issue?"Had the same issue with my gamepad. It turned out to be a bad motherboard. Hope that helps!
Answer to "Where can I get a motherboard for wii u game pad?"EBay is your best bet. They range in price from $35 and quick shipping (U.S. merchants) to $18 but very slow shipping, something like 10-25 business days (Chinese merchants).
Answer to "Gamepad is completely dead"More likely than not you're looking at a bad motherboard. If you press the Home button and the power button at the same time does the blue light turn on and then fade away shortly after?