How can I get apple security key

Apple doesn’t manufacture or market security keys.

Compatible Security Keys can be obtained from a number of sources. Some “high-end” computer stores may sell them. Most (if not all) of the common Security Key manufacturer’s sell directly, often via their own websites. Amazon and other online retailers are also good sources.

Security Keys can provide a useful additional level of security for your AppleID account in some circumstances - however, they are not necessarily a good choice for some (if not the majority) of users. They are certainly not for everyone. Be aware that if anyone can gain access to a trusted device - and have knowledge of your device Passcode - all protections offered by Security Keys can be negated in some circumstances.

In addition to all of your own devices meeting the required minimum requirements (iOS/iPadOS 16.3 or later), your trusted nominated Recovery Contact(s) must also have suitable Apple devices - although they do not necessarily need to be using Security Keys themselves.

You should note that if your Security Keys are lost, you are entirely reliant upon your nominated Recovery Contacts to assist with recovering access to your AppleID account. By design, neither Apple nor anyone else can access or reset your account; if you lose access to your account, loss of access will be permanent - along loss of all associated data - and all Apps and subscriptions purchased from the AppleID account.

You can find a list of supported Keys here:

About Security Keys for Apple ID - Apple Support

You must also have at least two (many would recommend three) compatible Security Keys - and this alone carries a cost.

The iPad and iPhone User Guides provide useful information about Security Keys and other aspects of your iPad and iPhone. Here is a direct link to the relevant section of the relevant Guides:

Use security keys to sign in to your Apple ID account on iPad - Apple Support

Use security keys to sign in to your Apple ID account on iPhone – Apple Support

You may also wish to read this topic which includes a nicely laid out case where somebody checked which methods were available to them with and without a security key.

How do security keys interact with other … - Apple Community