You get the dividers from your lfs, but they need to be really strong as these fish you have are big and robust. I have seen people use egg crate before and it works great. I would worry about your oscars though, as compared to a red devil they are weak, and as nature goes the weakest will be picked on. I would have all tanks put up and on hand for when aggression does come out.
My red devil really didn't show much aggression until he reached 5", then all hell broke loose and I lost 3 fish in one day because of him. But I do wish you best of luck with your guys, and hope you don't have aggression issues like I did and have to rehome them. I, to this day, miss my old red devil.