I have heard this applied to photography, writing and virtually any form of art.

Photography is a lot like prostitution..
First you do it for love...
Then you do it for friends...
Then you do it for money.

Where does this come from?

I'm always a liitle wary of internet attributions but I can't find it in any of my normal printed sources.

The common attribution on the internet has it as by Moliere and about writing.

Welcome, Captain Kodak. You'll be a great addition to our community!

While I agree that Internet attributions can make one wary, so can printed sources. Take my favorite word for example (epicaricacy). It has been printed in Bailey's Universal Etymological English Dictionary (1727), Novobatzy and Shea's Depraved and Insulting English, Mrs. Byrne's Dictionary, a Dickson dictionary of obscure words and perhaps in Shipley's Dictionary of Early English. Yet, many, including the OED, are highly skeptical of their entries. Even Wiktionary has deleted the entry for epicaricacy...twice; I've asked them for the reason for the deletion (I wasn't a contributor), but they've not responded. But my point is that all sources, printed or online, should be equally critiqued, don't you agree?

To quote Mark Twain (Sam Clemens) out of context: "Be careful reading health books; you may die of a misprint"

Bob

Absolutely.
Here's it's just that most of the sources citing Moliere as the author of this quote seem to be bloggers who have probably seen it on other blogs which doesn't really give it any air of great authority.

Wikiquote says that it's "attributed" to Molière, but I've been unable to find in which of his works it appears.

In French, it seems to be L'écriture ressemble à la prostitution. D'abord on écrit pour l'amour de la chose, puis pour quelques amis, et à la fin, pour de l'argent. Sometimes attributed to Molière. Mostly on blogs and quotes sites. Less than 20 ghits on the first sentence in the quotation (once you click past the first page).