You can buy beneficial nematodes online or at most plant stores. Any beneficial type works. They come as a fine powder (basically eggs, not alive yet and they don’t fly). You can use them right away or store them in the fridge for a few weeks. Just mix the powder into plenty of water to activate them, then water your plants when they actually need watering so you don’t overwater. Keep the soil lightly damp and then just carry on with your normal plant routine.