Isn't that the whole point? You don't need to get anything. It removes this whole "hmmm, you need to impress me company XYZ to get me to buy your console, they really need to impress me at E3, etc. etc. etc." stuff out of the way.
There probably are as many or more people on Resetera that can run Stadia right now without having to pay a penny on new hardware as there will be PS5 owners on this site.
One of the smartest things Google did was not mandating a streaming box or even having to buy the official Google controller. That takes the whole "hmm I dunno if I want to go to Best Buy or GameStop and drop even $99.99 on Google Stream Box + Google Official Controller" barrier to entry.
Anyone with a computer/lap top that can run a Chrome browser already basically has the hardware necessary. If you got a 25mbps internet connection and live in one of the available countries ... you have access to Stadia.
Now you'll just be watching a Youtube video and they'll probably give you the option to demo a lot of the games out on the service. There's no "gee do I want to invest in this console maker" dilemma really, they've removed all that.