I am no expert, but my first flush was amazing. I just juiced 2 large grapefruits and 2 lemons that I got at whole foods with a strainer.
If your buying a pre-bottled juice who knows what other fillers are in there and you might not get as good results. Grapefruit has a powerful natural cleaning effect and Im sure that it is used for the flush for a reason. Id go raw for sure, its easy to do.