The Home Depot, Lowe's, Grainger and SupplyHouse.com sell water heater drip pans in multiple sizes made from aluminium or plastic. Many municipal building codes require that a drip pan and drain line be installed under water heaters to prevent flooding in case of a leak or rupture.
Follow all building code requirements when installing drip panes and drain lines. Drip pans usually come with a PVC valve kit for draining. If there is no hole in the side of the pan, drill one of the appropriate size. Install the valve and connect either a rubber hose or a PVC tube long enough to reach a floor drain or the building's exterior, depending on the code requirements. Position the drip pan where the water heater can be set inside it.