In all the years that Google Fi (formerly Project Fi) has existed, you had to order SIM cards online from Google to activate your phone. Now, there's another way to get them. Just swing by your local Best Buy store, and you might be able to pick up a Fi SIM for a couple of bucks.
Google says Fi SIM cards are coming to more than 500 Best Buy stores across the US, but it doesn't have a full list (call your local store). Google ships SIMs for free, but maybe you don't have time to wait. Fine, you can pay Best Buy $9.99 for the same SIM card with no waiting. After you activate the SIM, Google will give you a $10 service credit to compensate you for buying the card. These are the standard voice and data SIMs—the data-only cards are still just available from Google.
Expanding to Best Buy makes a ton of sense right now. Google recently made Fi available on most unlocked phones, so it's no longer the only source for officially supported devices. Although, Google points out that Best Buy reps won't be able to help you with activation no matter what phone you have. You should be able to figure it out, though.