Your are going to have to build it if you want one that cheap but it will still be difficult for $100
You didn't say how big it needs to be.
Amazon has hydroponics systems and that will probably be your best online source. If are in an area where there is farming, the local farm supply may have something. There are specialty hydroponics places, like stealthhydroponics.com and hydrofarm.com, but were too $$ for me.
Building one yourself is quite easy. Look on the internet for system design and scrape the materials together yourself. Here's a video:
I need a vertical hydroponic growing system that's around $100.
