I had the hardest time finding them when I first started looking. I am not sure where you live, but here in California, most of the Raley's grocery stores have them. Sometimes Safeway will carry them but not always. I think it also depends on the time of year. I plan to plant a while bunch of dandilions in the spring! Way easier then trying to find them at the grocery store, plus it is cheaper.
Yep, same greens. I think the flowers are higher in sugar so you will want to feed those in moderation. The greens are a daily staple though. The greens you find in your lawn will be small, whereas the greens found at the store will be very long and full. Also, make sure there are no chemicals in the greens you find on your lawn. Fortunatly if you have dandelions growing on your lawn you probably dont have any type of fertalizer on it.