How can I get a remote for a used Apple TV?

The remote is essential. You should try to buy a matching replacement remote.

What is the exact model Apple TV box? This matters for the compatibility of the several models remote that are out there.

Identify your Apple TV model - Apple Support

Find your Apple TV model number - Apple Support

Various models Apple TV remote and their compatibility - Apple Support

Replacement remotes will need to be paired or linked to the box, else its signals will not get interpreted.

Pair the Siri Remote - Apple Support (Apple TV A1625 or later)

Link the legacy Apple TV Remote - Apple Support - Internet Archive (Apple TV A1469 or earlier)

Siri Remote for Apple TV A1625 or later can be purchased from Apple and from other resellers.

The legacy IR Apple Remote A1294 for Apple TV A1469 or earlier cannot be purchased from Apple anymore, but select resellers may still have some. Imitation remotes very similar to this model are also sold; not sure if those are any good, or have any issues.