First off, buy a good whey isolate protein powder. I recommend IsoPure Zero vanilla-25 grams protein per scoop, mixes in water with a spoon.
Then decide what flavors you want your shakes to be, and choose the appropriate DaVinci or Torino sugar free syrups. That way you just doctor the vanilla powder with the flavored syrup, and voila!--any flavor you want without having to buy a bunch of different powders. You can get both cheap at supplementwarehouse.com
Some people say not to buy a big tub of anything before surgery because your tastes change, but my tastes didn't change, and I don't complain that my protein shake doesn't taste like ice cream, like some people do. I mix my protein with water. Later, in your post-op liquids stage, this makes it easier to drink so you can get your 60 grams of protein in at the same time you work on your 64 oz of fluid. Some people mix their protein powder with soy milk, rice milk, or nonfat milk for extra body, but I'd rather not have the extra carbs and sugar. If you look around you can find low carb soy milk, but you have to read the labels. I look for 5 grams or less of carbs per meal. BUT my nutritionist says 15 grams carbs/serving from milk is ok--but I am so carb conscious that I don't follow that advice. I low-carb it for faster weight loss, and to prevent carb craving.
Once you get to the puree stage post-op, you can add frozen berries to your protein drink to thicken it. You'll lose more weight pre-op by eliminating those carbs, but it's ok post-op as long as you don't overdo it.
I see people on other sites adding bananas and other high carb fruit to their shakes, but then I remember--they are RNY Gastric Bypass patients--they don't absorb all the calories they eat, and as a Gastric Sleeve patient, I do. So be careful of RNY nutritional advice--they can eat more fat and carbs than us because they aren't absorbing it. We are.