You buy them from a supplier.

but to answer your question honestly the supplier keeps stock of them by growing them and storing them. The original cells came from donations.

You could always extract some cells from a biopsy as well, but these guys likely just bought them from sigma or whatever the Australian lab-supply monopoly is.

there are many "immortal" strains of cells that are mass produced and sold to labs, most notably "HeLa" cells. in this case in particular, "iPSCs" cells are used, sourcing skin cells or blood cells (with informed consent of the patient), and using those differentiated (or "specialized") cells to create pluripotent (or "less specialized") cells that can then be transformed into neurons