Check to make sure your current software doesn't have one. A number of programs have them built in. In fact, sometimes the biggest challenge is to get them to shut off. You could also use a drum machine if available or even record an old fashioned metronome. You might even try recording one yourself using a drum stick or even micing your foot on the floor as if it were a kick drum. Your sense of timing might be slightly off but with practice it's possible.