Regarding cooking time: not that most people have the time for it on an every day basis but for special occasions and/or really amazing polenta do yourself a favor and cook it for 3 to 4 hours over low steady heat. It's a completely different dish--you get some really lovey roasted/caramelized corn flavors you just never get on a quicker cooking time.
I picked up this trick in Bill Buford's book "Heat." Chapter 14 is more or less completely dedicated to anecdotal stories about cooking polenta. Much is made of the fact that, at Babbo, one of the biggest kitchen sins was forgetting to start the polenta early in the afternoon. Ideally, it was meant to cook for at least 3 or 4 hours by the time dinner service rolled around. Instant quick cooking polenta was on hand only in the event of an emergency, i.e, someone forgetting to start the day's batch early enough.
If you cook it this way, you don't need to stir it very much as it cooks--just add the occasional splash of hot water and give it the occasional stir as you go about the rest of your business in the kitchen. The abbreviated recipe (if you can call it that) from Buford in Heat is as follows:
"Polenta, cooked slowly for three hours, expands to about six times its original volume, so if you're making some for eight people . . . you want to start with about a cup. . . The amount of water doesn't matter, because you're going to add more than is worth measuring: you just want to make sure the water is hot so the cooking is steady. "
Obviously, the ordinary rule of thumb of whisking the cornmeal in quickly when it first goes into the boiling water still applies. The guiding premise to the cooking process from then on is to keep the polenta wet enough so that it looks like bubbling lava while it cooks. I should note, though, that if you add too much water too often, you'll reduce the opportunity for that all important roasting/caramalization magic to happen.
I usually finish mine by folding in a little butter, some parmesan or pecorino cheese and drizzling some good extra virgin olive oil over the top.
I agree with GWiv that Graziano's is the place to get it. Their Italian supplies are fantastic, second to none and a real bargain. I've found their cornmeal to be of a higher quality and better taste than anything else I've used. As a slight aside, lately I've been on a red lentil kick and the lentils they sell there are of unparalleled quality, as are all their beans, really. Jim (the proprietor) really knows his stuff and is incredibly knowledgeable and helpful regarding everything in the store. I usually go in there twice a month expecting to buy one thing and end up walking out with five or ten things. On that note, the last plug I'll put in for Graziano's is the olive oil they've been selling there. It's called Salvati and it's a beautifully intense unfiltered extra virgin olive oil. It's worth the trip all on its own.