In an ideal scenario, you always have more than one source of supply though in some cases you land up with a single source scenario. Seems like that is true for apple watch.
Mickey North-Rizza, VP Strategic Solutions at BravoSolution in a recent article on spendmatters.com higlights some of the challenges which may occur as the release date comes close to it. Seems like Quanta Computer is the only provider for apply watch. Though the article talks about apple watch, but I believe points highlighted by Mickey are generic enough to be applied across the board for supplier development
Close collaboration at the design stage to make sure that the supplier meet the critical design requirements.
Ensuring that the production is lined as per the various configurations you plan to offer. For example, Apple is offering watches from $350 to $17,000 for a 18 carat gold edition. Having configuration done at the later stages of the assembly will help vendor drive economies of scale and production efficiencies.
Close tracking of production schedule and ensuring that the production capacity can be utilized fully.
You can view the full article using the link below