Whence came the usage of the word "product" in cosmetology (specifically hair care)?

I don't know who first said it, but the Oxford English Dictionary has a little information about its background:

orig. N. Amer. Any commercial preparation used to style the hair or (occas.) as a cosmetic.

1989 St. Louis Post-Dispatch (Nexis) 27 Apr. (Style West section) 4 The key to making mascara work is ‘to make sure that there is not too much product on it’.

1993 Chatelaine (Electronic ed.) Nov., Two of the most common mistakes people make with gel are using too much product and applying it incorrectly.

2001 N.Y. Mag. 25 June–2 July 58, I don't wash my hair or even rinse it after the beach—I just put a lot of product in to make it shiny.

2004 S. Olson Children of God go Bowling iii. 44 There was just the right amount of product in his hair to make it look both coifed and neglected.

So, the first published usage the OED knows about is 1989, but it was used there to mean mascara, not a hair care product. It was used for hair care products by 1993. (You could argue that in the 1989 and 1993 citations, product is actually referring back to an earlier noun, standing in for mascara or gel to avoid awkward repetition; in that case, the earliest OED citation that definitely uses product all by itself to mean hair care product comes in 2001.) The usage seems to have started in North America.

Anecdotally, the first place I heard the word used this way (I am not what you'd call fashion-forward) was on the TV show Queer Eye for the Straight Guy sometime around 2003 or 2004. After that, I started hearing it everywhere. Clearly they weren't the first to do it, but they may have been the ones who brought the term into the mainstream. (Then again, what's more mainstream than the St. Louis Post-Dispatch? I may need to rethink this theory.)

One more reference: on the animated TV show Drawn Together, in the episode entitled "Gay Bash," God appears as a character and says,

I love the gays! Come on, they're adorable! And calling hair gel product? That's just fancy, that's what that is!

The episode aired in 2004, at the height of Queer Eye's popularity.

I believe "product" comes from the broader term "hairstyling product" and has just been shortened.

I just used some hairstyling product on my hair and it looks great!

This would naturally progress to:

I just used some styling product on my hair and it looks great!

Finally, we reach the present day:

I just used some product on my hair and it looks great!

This seems to make the most sense, though a concrete source on the matter does not seem to exist.

Have a look at the following links on the history of hairstyles (esp. the second link).

History of Hair Styles

Hairstyle History

Not sure how credible these sources are but, according to the information provided, the term "product" appears to have entered the vocabulary of hairdressing as far back as the early 1900's. Unfortunately, one has to subscribe to the website of the second link in order to obtain any further information.

The word product is used in that fashion in all industries and distribution chains that deal with any kind of product. In the soft drink industry they refer to their beverages as product, in the back rooms of many grocery store they have "product" the same way that milk companies have "product" in their warehouse.

It is easier faster way of dealing with what could be hundreds of different actual products, the usage is becoming more general in terms of who is using it in this way, its just natural evolution of the word usage.