240v is actually two phases of 120v, so if you carry a neutral (the white wire) along with your circuit, you can make (2) 120v circuits out of a single 240v circuit. That is going to be the most efficient method, and perhaps the most cost effective if you haven't run the circuit yet or there is a neutral available. Amazon carries some transformers that are pretty cheap, but any transformer is going to have some losses associated with it. If you need to use a transformer, the price is going to go up pretty quickly with increased capacity, so sizing it reasonably closely to your needs would be a good idea. However, they need plenty of extra capacity to get the loads started.
Using a neutral with your circuit is by far the best method.
You need to make sure that the lights are on different circuits, not just separate receptacles, and verify that both of the circuits are running pretty close to unloaded.
Plug a light into whatever receptacle you want to map first - if you have a drop light with a cord long enough to reach the breaker panel so you can easily see the light go off, it will make it easier. A 120v-powered radio that you turn up loud enough to hear at the panel also works well.
Now, trip the breakers one at a time until you turn the light or radio off. Leave the breaker off and go see what else in the house went off at the same time. It is very unlikely that the single receptacle is the only thing powered by the breaker, so you need to verify what else is on the circuit.
Now do the same thing all over again with another receptacle that is within reach of your grow.
When you have gone through this to the point that you have found two receptacles on separate circuits which are minimally loaded, you should be in good shape.