Where do I purchase the YubiKey + LastPass Premium bundle?
You can purchase one of the YubiKey + LastPass Premium bundles on the Yubico website: https://store.yubico.com/.
If you already have LastPass Premium, your Premium purchased through the YubiKey bundle will simply be added to your existing Premium. If you had one year, and you purchase a year with the bundle, your Premium will be up for renewal in two years.
You can also learn more about the YubiKey here: https://lastpass.com/yubico/