ETH can be purchased in several different ways:
You can click "Buy ETH" located at the top of the 1inch desktop site. This will take you to the 1inch 3rd party payment processor, MoonPay, where you can purchase ETH with a credit/debit card.
You can also buy ETH or other tokens directly within the 1inch Mobile Wallet.
If you already have a little bit of ETH, you can swap for more right here on 1inch.
Also, if you only need a small amount of MATIC, you can get some here for free!