Home improvement retailers such as Home Depot and electrical specialty stores such as Batteries Plus sell replacement light bulbs for use in microwave ovens. As of 2015, they cost between $3 and $5 and provide a brightness of 200 lumens while using 25 watts.
It is helpful to take the old bulb along when buying a replacement bulb to ensure a good match. Comparing this bulb with the available stock in the bulb store eliminates the need for return trips. Knowing the manufacturer and model of the microwave oven can also ensure purchasing the correct bulb. The associates in the stores can provide assistance, and the specs on replacement bulbs should also appear in the microwave oven owner's manual.