Like in this sentence:
Fashion is not ________ . It's made.
Obviously I can not write "made" in the blank, because it would imply an involvement of some external factor. I want a word which refers to create itself on its own or by itself.
"It's made" implies that some external thing like humans or designer are responsible for making it.
The word you are looking for is born.
Also you can try found, given, or taken - none of these mean "self-made" but they do refer to a transfer of something X to you, versus you making a new X.
A third suggestion is to use the idiom pulled out of thin air.
There is no common English word that conveys the meaning you want. (or at least, none that I can think of right now)
I've offered below two phrases that are relatively common (and essentially idiomatic) that come close to what you want, but there may well be others. It is always very hard to suggest options, when the context is somewhat figurative.
Fashion doesn't just make itself. It is made.
or
Fashion doesn't just appear out of thin air. It is made.
It's a bit of an obscure answer but you could say:
Fashion does not exist ex nihilo.
'Ex nihilo' is a Latin phrase meaning 'from nothing'. Usually it's used in a theological context (e.g. discussing God creating the world 'from nothing' in a biblical context), referring to the idea of the universe being created 'ex nihilo' (from nothing). By using the phrase 'ex nihilo' you are giving emphasis to the fact fashion does not just 'come into existance (from nothing)'.
If you are looking for a word that means something like without "involvement of some external factor" you might want to try spontaneously. From Oxford Dictionaries:
1.1 Without apparent external cause or stimulus.
The adjective, spontaneous, can actually mean exactly what you want:
1.2 (of a process or event) occurring without apparent external cause.
So you could, theoretically, plug it right into your example. Unfortunately, this is not the most common meaning of the word, and the sentence would not sound quite right. On the other hand, a much more common and unambiguous way of saying this would be to use the adverbial form of the word:
Fashion does not arise spontaneously. It is made.
The word or phrase you should pick will depend on the context that you want to use it in.
If you want it to mean exactly what your question states (self-created) and are using it as a single-sentence explanation somewhere, there isn't a single, concise word that I know of that will do this. You will have to pick a phrase that (I think) will sound clunkier, and the best phrases use the auxiliary verb do, not be (is), which ruins the symmetry between your two sentences.
If you are intending to use it as a piece of poetic language to more stylistically get your point across, e.g. as a slogan, or as part of a longer essay where the sentiment is explained more fully, then you can use language that doesn't mean exactly what you want say, but carries something of the correct connotation.
If the former is the case then you could use the phrase "just appear" (or "just appear out of thin air", or "just appear out of nothing"). The use of "just" gives emphasis to the effort required.
Fashion doesn't just appear. It's made.
If the latter is the case, and especially if you think your audience has a significant Christian makeup, then you could use the word begotten. Begotten essentially means "fathered", so carries the same sort of meaning as "born", but one of the most famous uses of the word is in the Nicene Creed (which many Christians will know) in the line "begotten not made". Saying the opposite of this famous quote would give it some poetic style. That said, the word is "church language" and quite old-fashioned, so it could sound odd to some people.
Fashion isn't begotten. It's made.
"Fashion does not just spring into existence by itself."
That is a common way to say it.
The problem with using born is that it's actually incorrect. Merriam-Webster says that born means "deriving or resulting from". Suggesting that it's not born is contradicting the second statement.
Fashion cannot exist before it is born by the creative mind. -or- Fashion doesn't exist before it is born by the creativity of man.
You could also say that fashion is not self-born.
Being that I'm not sure exactly what the intent of the statement is, it's difficult to give an exactly appropriate answer. If your statement is only an example and you're really seeking to convey the meaning that it doesn't create itself, self-born would be appropriate.
Addressing your example as directly and closely as possible, I would suggest "fashion is not self-born, it is manufactured" i.e. fabricated.
Although the best phrase to describe the concept you're asking about (spontaneous generation) doesn't fit very well in the example sentence, there are a number of other phrases not directly related to creation that may work instead. In addition to those already presented on this page, another option would be "in a vacuum".
Fashion doesn't come out of a vacuum, it is made.
Fashion doesn't arise in a vacuum, it is made.
"In a vacuum" means that something isn't affected by external factors (1) (2).
Or simply "on its own".
Fashion doesn't exist on its own, it is made.
Fashion didn't arise on its own, it was made.
"On its own" means without the help of others.
I have not seen this mentioned, but I would use the word 'Instantiation' so;
Fashion is not self-instantiating. It's made.
Though this is probably more a technical term from the Software Development world.
The tecnical term to describe if something creates itself out of itself (or parts of itself) is Autopoiesis. It was introduced by the Neurobiologist Humberto Maturana in the 70´s. For example during mitosis a living cell reproduces itself out of itself. The term is not commonly used. It is not suited to answer your question.
But a term that is often used in this context could be useful: Emergence. Fashion doesen't emerge, it is made.
