https://www.tesla.com/blog/panasonic-and-tesla-sign-agreement-gig...

https://www.tesla.com/blog/panasonic-and-tesla-sign-agreement-gigafactory

Taken straight from their website
"According to the agreement, Tesla will prepare, provide and manage the land, buildings and utilities. Panasonic will manufacture and supply cylindrical lithium-ion cells and invest in the associated equipment, machinery, and other manufacturing tools based on their mutual approval. A network of supplier partners is planned to produce the required precursor materials. Tesla will take the cells and other components to assemble battery modules and packs. To meet the projected demand for cells, Tesla will continue to purchase battery cells produced in Panasonic's factories in Japan. Tesla and Panasonic will continue to discuss the details of implementation including sales, operations and investment."

So it's not where will tesla get it's lithium supply. It's where Panasonic will get it's lithium supply to supply battery cells to Tesla. At this stage anyway.