For anyone who wants to complete every aspect of their Pokédex, the Vivillon addition to Pokémon Go offers a challenging goal. There are 18 distinct versions of this Pokémon, and the way you unlock them comes from exchanging post cards with friends.

Of these 18 unique forms, two are proving to be the most challenging to unlock. The ones proving to be the most difficult Vivillon forms to unlock are the Ocean and Sandstorm variations. Each type of Vivillon is connected to a specific part of the world, and you need friends to send you postcards from those locations. Here’s what you need to know about how to get the Ocean and Sandstorm Vivillon forms in Pokémon Go.