The lucerna, "Loo-KAIR-na", or sacred lamp, was most usually the source of sacred flame at the Lararium altar. This was an oil lamp made of clay or metal that was lit during the rites in honor of deity. There are companies which make reproductions of ancient Roman oil lamps, but one can use a small modern oil lamp just as well. It is the flame that's important, not the container. Even more easily, a white votive or taper candle in a holder may be used. The tallow candle was invented by Romans and certainly was used in ancient times. A lucerna or candle should be on the lararium altar always, but it needs only be lit during the rites or when an offering of food, flowers, etc. is made to the gods.
Before statues were used in the household rites, the home altar centered around a sacred fire. This fire was a representation of the goddess Vesta, but also it was a combination of offering to the gods, and a representation of the power of the gods. In all eras of the Religio Romana, a sacred flame was part of household worship.