Walmart, and it's $2 & some change per pound, they don't always have it but some stores do during pickling season, I've managed to find it at the Union City location a few times so I bought a bag or 2 every time I was there. I've used kalkwasser for a year now and I probably used less than half a bag.
I don't used fully saturated kalkwasser, it gums up the tubing in my peristaltic pump so I do 2 teaspoons/1.5 gal of RO water + some vinegar, I dose it for 1 minute out of every 10 minutes throughout the night while lights are off, now this isn't a formula you want to follow for your tank since your chemistry is different than mine.
If you're going to try kalkwasser I recommend you read this;
http://reefkeeping.com/issues/2005-01/rhf/index.php
Then read it again, simply because Kalkwasser can be great but it can also wreck your entire tank if it goes wrong. It is not recommended to put kalkwasser in your ATO but I'll admit I've done it, I would say do not dose it this way unless you've exhausted all other options and you have no choice, ie; no room for equipment or if you have no sump. Now that i have a sump, first thing I did was separate my ATO from my kalkwasser this way no disasters are going to happen. Also it helps if you have a controller that monitors PH because you can set it to shut your kalkwasser off if PH gets up too high while you're dosing (very good fail safe). I can go on & on talking about kalkwasser and many other variables that come with it but it's best to have your own understanding of the chemistry taking place here, that article is better than anything I could fit in a post here
