You don't need to go out and buy hardware keys today. You should be able to use any FIDO2 key these days for SSH, as long as your server runs a recent enough OpenSSH server, which supports ecdsa-sk or ed25519-sk public keys.

Reputable vendors of those hardware keys included Yubikey and Token2, among others. You are really just using them as FIDO2 tokens, so go for what's reputable there.

If you are just getting started, you can generate and use software-backed keys (using ssh-keygen) and copy the public keys to servers' authorized_keys file using ssh-copy-id (or manually adding it). You should probably start learning this way (with plenty of online guides, and fewer risks of an old server implementation not supporting FIDO2 credentials), and then move to hardware keys once you understand the mechanics of copying around public keys, etc. Your password manager might also support acting as an SSH agent for key based logins - I believe Bitwarden and some others now do this, for example.