Julie - in theory, your surgeon/NUT should provide you with information on a pre-op diet. Ask about it at your next visit.
Some surgeons (like mine) did not require a liquid diet. I was just required to lose 10-15lb and have a low fat diet. Ultimately I lost 35lb so I must have done something right.
The norm seems to be 800 to 1200 calories per day, 70-100 grams of protein and a lower intake of carbs and fats.
Until you know what your surgeon requires, I wouldn't rush out for protein supplements at this stage.
But when you do need them - I'd recommend you get a selection of sample sized baggies cause trust me - protein supplements are very much a personal choice and you can't guarantee on like the same stuff post-op as you did pre-op.
Best selection I know of is from vitalady and all their products are of high quality and suitable for us:
Vitalady.com - Protein Supplements, Vitamins & Minerals, and Safe Snacks!
Syntrax Nectar is good stuff and the good thing is that they do a range of fruity stuff and some milk shake ones too (I like the milk shake ones best).