You won't find many places that will teach actual tactics in open-enrollment courses. I have been thinking for sometime about putting together an actual tactics course for CCW holders that would use scenario based training, force on force, etc. You'll find a few places that will give you a taste, but not many. VTAC street fighter, some TFTT courses and a few others here and there cover a few things (fighting around vehicles, some small unit movement and communication), but for the most part what you see out there are marksmanship and weapon handling classes.
As for the cover discussion - it stops bullets, opposed to concealment which just hides you from sight. People like to get sucked up real close to a piece of cover because it makes them feel safe and secure. As a general rule, you want to keep back from it a little - it will give you a better view around it, and you'll be less likely to catch ricochets off whatever your cover may be. The exception may be when you are using said cover as a barricade to support a shooting position.
Using cover to your advantage is a game of geometry. You try to put yourself into a "position of advantage" by seeing your opponent before they see you, and keeping yourself from being exposed to fire, while forcing your opponent into the open. If you can put yourself into so much of a better position than your opponent, it will be nearly impossible for them to win a fight. This doesn't just apply to cover, but speed, position, firepower, surprise, etc. I've been in "fights" that were concluded without a shot being fired - where a suspect gave up and later said the only reason they didn't go for their gun or shoot is because they knew they were at such a disadvantage from the start, and wouldn't survive.
You can "practice" your movement and use of cover when you're out and about. When you are moving around a corner, aisle, vehicle, at WalMart, in your home, etc... look for other people. If you see them before they see you, you win. If they see you first, you lose. Play the "what if" game and consider where you would move to, what you would do if someone walked through the door of wherever you are with a gun and started shooting. Situational awareness is the first step to using cover to your advantage.