I have recently been able to openly discuss this issue with a highly reputable hydroponics ditributor. I gained the following knowledge on the topic: although not widespread...There are some, if not several, online hydroshops, and even some local, I guess...but, several online hydro shops that are set-up by individuals that took a chance being in the hydro business and grew Pot; And, got busted...so there are currently, some shops out there to be wary of.
here are a couple guidelines that I feel will help you if you buy online. I do once in a while, but now I have a distributor that I deal with exclusively.

1. do some research and find a reputable hydro supplier. I recommend a "real" store, not a shop that is merely an online business. A real store on the street somewhere. Leo is not sitting across the street from your local hydro dealer, unless he grew/grows pot and got busted or is under investigation.
2. don't ever mention what you are doing...they know, basically. If you have to ask for info...do so in a general manner. 2 important things to know is. a}most veggies do not require change of photoperiod in order to produce fruit...So, don't ask about when to change the photoperiod, or anything along those lines. And remember. tomatoes, peppers and cucumbers are all grown using similar methods to those used in the cultivation of mmj.
b}they will cut you off, if you make your illegal intention's known...that denotes conspiracy and put's their business in jeopardy...

I have some more to say on the subject, but my short term memory forgot...peace talk at ya' later.