If you are referring to buffers then anyone's will do. I too find the capsules convenient and very much like the idea of having freshly mixed buffers for each use. You can get the capsules from Cole Parmer or Hach.
If you mean the electrode storage solution it is best to get that from the manufacturer. It's probably going to be saturated potassium chloride but if some manufacturer wants something special then getting it from him insures that you are doing what he recommends.