How can I make options within a field available or unavailable please?

We have products that are not always in stock and would like to change the options in a down menus, according to their availability

You can use the Condition logic tool to Enable/Disable an option/s base on conditions rules.

User guide: Smart-Forms-Using-Conditional-Logic

Example:

Here is a test form https://form.jotform.com/61146783791969

Let us know if you need further assistance.