Where are the 16gb DDR3 sodimms?

This is becoming annoying. I know there are laptops out there, Clevo, Eurocom, Dell Precision had them that suppport 64gb of ram.

The only way to do this is with 16gb sodimms.

1. Where are they?
2. What do they cost?
3. What determines if you can use them? (processor, north bridge/south bridge, bios)?