There are several tools you could use to find this, but a free and reliable one is PHI Latin texts, which allows you to search for strings of characters within an author (or authors).
Knowing that the preposition ad takes an accusative, you could just search for all instances of ad in Cicero, which will lead you to find very early on the neuter noun vadimonium. This is an unambiguous example, then, of a neuter noun in the accusative (your earlier wording).
For neuter direct objects, you'll want to find a sentence with a transitive verb and a non-neuter nominative or, perhaps easier to do, a plural verb with a singular neuter. In that case, pick a neuter noun—any will do—and put it into PHI to search for it near "nt#", which will likely give you what you're looking for.
I chose nomen (wanted a different example from vadimonium and of course a clear and unambiguous neuter noun). Lo and behold, we have an answer:
...hos digitos meos impellere potui ut falsum perscriberent nomen?
...am I able to impel my fingers so that they write out a false name?
In this example, the ut-clause has only three words within it (aside from ut), an adjective, a neuter noun, and a plural verb. In this case, it's impossible that nomen falsum could ever be the subject of perscriberent. It's thus a clear and unambiguous example of a neuter noun accusative because it's a direct object.
