You can store MKR in any cryptocurrency wallet that supports ERC-20 tokens on the Ethereum blockchain. You can use a browser wallet like MetaMask or MyCrypto, or download a desktop or mobile wallet. Some examples of cryptocurrency wallets that support MKR include:
Trust Wallet
Exodus Wallet
Atomic Wallet
Guarda Wallet
If you own a significant amount of MKR and plan to hold it for a long time, consider buying a hardware cryptocurrency wallet such as a Ledger Nano S. Hardware wallets offer a very high degree of security and prevent your private keys from being exposed to attackers.