I have no idea where you can find it in "real life." There are a lot of websites that sell chemicals, they'll usually have it. I actually bought a pound of it from ebay!
ebay search resultsI got the powder kind, because it makes more solution and will last longer. But I've also seen concentrated solutions of it for sale, those are probably less messy.
As long as you rinse your plants in clear water for a few seconds, you'll be fine. The PP at a low concentration doesn't hurt your fish, so it's ok if the plants might have a little bit on them still.