Instead of grapefruit juice you can try lemon, lime and probably also pineapple and orange juice or mixtures of the above. Of course, they all need to be made by yourself freshly from the fruit. I doubt that there are any commercial juices that may work. Lime should taste excellent and will surely give a nice mixture. With the others you may want to experience with small amounts to check for taste and mixing properties before the flush. The “textbook” emulsion obviously doesn’t work for you but there are plenty of alternatives. Let us know how it works out.
As has been pointed out you may want to start with maybe only half a cup of oil + half a cup of juice. After managing that you may increase the amount with further flushes.