You can buy swarm lure from Mann Lake, Brushy Mt etc.
But Lemongrass essential oil works just as well and is cheaper. Both will work better than just old comb, but old comb will help. Lemongrass essential oil smells like Nasonov pheromone. Just use a few drops (three or four) in the bait hive.
You can also drop your old queens in a jar of alcohol and use the alcohol for swarm lure. The alcohol has the QMP (queen madibular pheromone) in it. Just use a few drops in the bait hive. Works well in combination with the Lemongrass oil.
Of course, bait hives are like fishing. It's a gamble. If you find a good location I would put more there. If you aren't getting any swarms by the end of the swarm season, I'd move them to another location next year.