How do I attain a physical security key?

Apple does not sell or provide physical security keys; however, do they provide examples of FIDO® Certified security keys that work with Apple ID:

• YubiKey 5C NFC (works with most Mac and iPhone models)
• YubiKey 5Ci (works with most Mac and iPhone models)
• FEITIAN ePass K9 NFC USB-A (works with older Mac models and most iPhone models)

You can learn more here: About Security Keys for Apple ID - Apple Support

To meet the current technical requirements of Apple’s Security Key implementation, you’ll need at least two (2x) FIDO2 security keys.

The keys cited in Apple’s article DO meet that requirement - and a lot more - but there are also other FIDO2 hardware keys available which will function equally well.