Hi Tommy
If buying for your pens or to sell on to your customers then Phil is your man but if just giving information to your customers who usually ask where they can get refills, then the usual stores such as Staples, Ryman, Smiths etc is where they can go. I've also seen them is Tesco or Asda just can't remember which.
You should maybe encourage them to come to you if possible though if you want to make some extra profit.