MikeVV,
Yes, there are many off the shelf injector units like this available through greenhouse and hort suppliers, as well as some companies that sell animal medications which are put in the water. They range greatly in price and style, depending on the flow rates and accuracy you need. Most put a liquid or slurry into water, not many handle solids.
Also, you can make these pretty easily if you are handy with things like PVC pipe.
If you need specific leads, let me know. Dosematic and Dosatron are two that come to mind, but there are many others.

As regards injectors - I have always been pleased with Blue-White brand injectors. Simple to use and can handle up to 100psi. We used them on several city irrigation lines to inject catalytic enzyme and nitrobacter to clean out biofouling of sprinkler systems - stopping daily replacement of Rainbird heads. Goggle search under "BlueWhite and you'll find them. Typical 30-gallon unit with built in injector pump runs about $500 or so wholesale.

Dave/Aquatic Technologies

I can vouch for the doseatron hydraulic injectors. We use one large one for injecting nitric acid at our nurseries and have been for years.
They are reliable, easy to use and have the advantage of being non electric have relatively few things that can go wrong.  
We changed to this from an elecric injection system when this broke down. The downsides are loss of accuracy (not that relevant to me anyway) and noise.
Hydraulic injection is MUCH noisier!