1. If its a standard filling loop, any 1/2" BSP end cap will do. E.g. Screwfix 51939. Make sure it has rubber or fibre sealing washer inside.
2. If its the Vaillant built in one with two grey plastic handles, you would need to measure the threads (diameter, thread pitch) to stand any chance, as I don't think Vaillant list anything suitable as spares.