Pokémon Shirts is an online service that allows you to customize your own original shirt freely using a combination of patterns based on Pokémon. In celebration of the release of a new set of patterns based on the Pokémon who appeared in Pokémon: Gold and Silver, 4 new patterns have been added to the Style Shop in Pokémon GO! Look out for shirts based on Ledyba, Skiploom, Smoochum, and Unown.
For more information on Pokémon Shirts, you can check out the homepage here: https://pokemon.originalstitch.com/
Physical Pokémon Shirts merchandise is currently only on sale in Australia, Canada, France, Germany, Hong Kong, India, Indonesia, Italy, Japan, Malaysia, New Zealand, the Philippines, Singapore, Spain, Taiwan, the UK, and the United States. Check out this link for more info on Pokémon Shirts. Happy exploring!
—The Pokémon GO team