Usually special items are dropped from killing monsters and opening stuff throughout the journey, but I was wondering if there's also a slight chance of them being spawned from Griswold's list of premium items for sale.
Unique Items in Diablo 1 are restricted to being dropped by monsters within the Cathedral and its depths.
Unique items have a quality level which is largely based on the item type. Monsters must have an Mlvl high enough to drop an item of the unique's Qlvl to spawn that unique. This isn't much of a problem in Diablo or Hellfire, since most uniques have fairly modest Qlvls. It becomes a problem because which unique item is generated is based on the monster's Qlvl, and due to poor coding this makes a number of uniques almost or completely impossible to find.
Basically, unique items rely on NPCs that have an "Mlvl" value - which is restricted to the monsters in the dungeons. Griswold does not have an "Mlvl" anywhere in his code, making it impossible for him to generate unique items for you to buy.
Monster level is a fairly important function in the Diablo games as it pertains to item generation and drop tables.