What do you mean by multi-step authentication? Is that the same as two factor?
A 'factor' is something you own, something you are, or something you know. Using a password and a Yubikey would be one
Can I use two-factor authentication with the Basic subscription?
Yes, if you choose to log in with a single credential type that uses two factors. The Yubikey With PIN, Password With IP Address or a smartphone credential would count as two-factor. You won't be able to use two login stages to combine two different credentials unless you upgrade.
Where can I get a Yubikey?
You can order Yubikeys from the Yubico Store