Technically, this is incorrect. The very first officially released Pokémon product are indeed the Japanese GameBoy Games, released on February 27th, 1996.
But, the first versions of pre-release flyers for those games were released on November 23rd, 1995, still mentioning the originally intended release dates of those games of December 21st, 1995, until they got postponed (and a second version of this pre-release flyer was made, mentioning the correct February 27th, 1996 date).
There is also the book New Game Design, written by Game Freak’s founder Satoshi Tajiri, which released in December 1995. This book contains some early beta sprites of the upcoming Pokémon Pocket Monsters games, and also depictures some on the book cover.
Likewise, there is a Famitsu magazine volume of November 17th, 1995, which previewed the games that are going to be shown at the Spaceworld event, including Pokémon.
The volume after that of this same magazine also shows some, which was released on December 1st, 1995.
But, going even back to December 1992, a Happy New Year 1993 post card was given internally to Game Freak employees, which depictures early designs of Exeggutor and Kangaskhan:
FYI: The first Pocket Monsters beta sprites were designed in ~1989, and the original name was Capsule Monsters, until that was postponed more than once and later rebranded as Pocket Monsters. Physical copies of those early Capsule Monster concepts only exist internally within Game Freak, like this one that was shown during a Japanese TV show in 2018.
Sources:
- Most are answers in this thread I create: First physical product to mention/depict Pocket Monsters?
- HelixChamber.com, containing great information about early Pocket Monster game sprites/maps, and the early Japanese history of Pokémon.
But, going back on topic for the first released Pocket Monsters cards: see the article by @rainbow in the comment above:
Bandai Carddass Part 1 were the first released Pocket Monsters cards, released on September 21st, 1996.
PS: The Bandai Club Part 1 stickers could also somewhat be considered cards imho, released in May 1996.
Greetz,
Quuador