A couple of possible products that work well on granite - Premium Stone 7 Grout Sealer from HydroForce or Heavy Duty Sealer by Stonetech.
For walls, you can apply with a clean sponge and wipe it onto the surface. I suggest two coats. Because of the humidity, heavy use, water, often lack of proper cleaning, sealer on a shower wall will almost never work effectively as long as it will on a floor.
Scott Warrington