xDai direct purchases with Wyre are currently under construction. You can use Ramp with Euro integration or purchase Dai on Ethereum and use the Bridge UI to get xDai.
Prior Portis Instructions (not currently active)
Billing addresses in the US use the Wyre API to purchase xDai with Portis. US buyers are limited to $40 per day and pay a fee of ~3%.
1) Go to portis.io and click the Get Started button to create a new wallet (or login to an existing wallet if you have one).
2) You will see xDai in the tokens list. Click on the Buy icon in the bottom menu. If you have not verified your email, you will be prompted to verify.
3) Enter your details in the required fields. These are not needed for crypto wallet functionality, but are required for xDai purchases.
4) You will be sent an SMS code. Enter it into the interface.
5) Click to Add a Debit Card.
6) Enter in your card details and click the Add button.
It may take up to 48 hours for approval.