If you have an account at an exchange already (and they don't offer XMR) the easiest thing to do is to buy something like BTC or LTC and exchange it via a third party to XMR. You can both make the exchange to XMR and store the BTC/LTC/XMR in Cake Wallet on your phone.

If you don't have an account on an exchange yet, you can make one on Kraken (and live somewhere you can) and buy XMR with fiat and directly withdraw it to Cake Wallet or whatever XMR wallet of choice you have. I personally like Cake because they make it easy to exchange from LTC to XMR right in the app and had built in gift card purchasing (temporarily disabled).

A third option would be to forego the use of an exchange and use https://localmonero.co/ This may actually be the easiest option depending on what country you live in. There are local monero sellers that take everything from PayPal to Western Union transfers so you should be able to find something that meets your needs.

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