How can I add items to Find My without an AirTag?

To be able to add Items you can track, you must open the Find app, tap Items in the bottom menu, then tap Add Item, and then tap Other Supported Items.

You must be aware that the accessory you are trying to add must be marked with Works with Apple Find My.

You can find a list of the 3rd party accessories that are currently supported:

Keep track of personal items using Find My - Apple Support

What item do you want to add? You can track any Apple device (iPhone, iPad, iPod Touch, Mac, Airtag, AirPods, Apple Watch…) with Find My from an iPhone (or any other Apple product with Find My) or from https://icloud.com/find. And you cannot track any item that is not made by Apple.