There is an example or a restatement (or an original statement) of that here, in the Uposatha Sutta,
 And furthermore, just as the ocean has a single taste — that of salt — in the same way, this Dhamma & Vinaya has a single taste: that of release...
Bikkhu Bodhi quotes that passage in a couple of his essays, e.g. The Taste of Freedom and The Living Message of the Dhammapada.
There are a few passages where the Buddha is quoted as saying,
Both formerly and now, it is only stress that I describe, and the cessation of stress
... which I think is similar.
You may be reading too much into the translation. You're reading "the law of one form" and think that sounds like "the law of gravity" or "the law of Newton", etc. (i.e. a specific named law).
But "law" in that context is presumably "Dhamma" or "Dharma", so "the law" is "the Dhamma".
And "the law of one form" might be translated as "the Dhamma which has one form" -- so I suppose it's talking about the doctrine of nirvana -- that's why you couldn't find it on the internet, because "the law of one form" isn't a standard phrase/name for the concept.
Part of the reason why "the law of one form" is not a very standard concept or phrase might be as follows. When you consider "the law of gravity" or "the law of thermodynamics" then what's being described is "gravity" or "thermodynamics". But the phrase "the law of one form" isn't a description of "one form", rather it's a description of "the law" or of "Dhamma". It's not saying there's something important to know about "the one form", instead it's saying that there's something important to know about the Dhamma ... and the important thing to know is that the Dhamma has one form (or perhaps another sutta might say, "one direction") i.e. it tends towards nirvana, its purport is the cessation of stress.