Understood. Currently the only publicly available options to obtain a code is either purchase a Premium edition, or purchase a Chromecast Ultra with an offer that includes a time limited free subscription, or any available hardware (phone/tablet) that includes a subscription, or a buddy pass from someone that purchased a founders edition. Also some ISPs are offering access too. Else the only option would be to wait until the base edition is released at some point in 2020.
I’m not sure if there are any other options available beyond that currently, Stadia is still in it’s somewhat limited distribution mode at the current time. Hopefully for you and others it will change in the not too distant future, but the only option for the majority not willing to spend money on new hardware or ISP contract is to wait until then.