There are many retailers online which sell keysets, especially for 'standard' cross mount key stems. If you are lucky, your keyboard will have come with some spares in the box. For the two mechanical keyboard in the writer's possession, both came with a selection of extra blank keys – not ideal, but better than a missing key. If your keyboard is quite old, but you are fond of it, it might be worth grabbing a cheap used one from your local auction site(s) – then you will have spares.