Monstro City is a claimed NFT collection, which means that the NFTs of the collection are not sold by the artist but can only be claimed if you've met the requirements.
As of now, the requirement is set to owning an NFT from "The Dimension That Never Existed" collection by Sarp Süerdaş.
Monstro City is limited to 34 items and it is first come first serve basis. So if you have collected a piece from TDTNE but there are no Monstro City items left, good riddance, you better be faster next time :)
Note: If you think you are eligible to claim a piece from Monstro City, please get in touch with me via Twitter @sarpsuerdas