Lost my air tag can I get a free replacement

Nope. That is not an option. Is the AirTag still showing in Find My? If not now, it may appear when another Apple device is within Bluetooth range. They are only $29(US), probably cheaper than recovering from your vacation spot when it does show up.

You can buy a new one. Apple won't give you a free one though.

There is no loss coverage for AirTags.