Where do u get apple security keys from

You can purchase YubiKeys from https://www.yubico.com/store/

Another source is Amazon: https://www.amazon.com/yubikey/s?k=yubikey

Also, see the following from About Security Keys for Apple ID - Apple Support

Security Keys for Apple ID works with any FIDO® Certified security key. Some good examples include:

• 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)

If you choose a different security key, you should choose security keys that are FIDO® Certified, and have a connector that works with the Apple devices that you use on a regular basis.

For a complete list of certified keys from the FIDO Alliance, visit the FIDO® Certified Showcase.