We have different customized forms for Invoices. Each customized form has their related Advanced PDF/HTML Template. Now, for scripting purpose, we need to get TEMPLATE ID value as we need to send it as parameter in SuiteScript. Any idea how to find that ID?

Custom Transaction Form isn't scriptable. You'd need to store that template ID somewhere else and access it.

potentially use render.transaction instead to render the template associated with the form