Sometimes you can find them at your farmers' market or specialty grocery. Bundled and placed next to the other expensive greens, they make a fine treat, but put a real dent in your pocketbook at $5 a bunch. Or you could just stuff a plastic bag in your pocket and go out for a walk. I guarantee that you will find some dandelions before long, poking up in your neighbor's lawn, clumping in green queues next to the sidewalk, or squeezing through a crack in some vacant lot.
If you intend to eat your dandelion, though, I wouldn't go searching for it in a nearby yard, unless you know for sure that it isn't sprayed with all sorts of poisonous lawn chemicals meant to kill the very plants you are out to collect. Find a dandelion that hasn't been assaulted by human kind, one that doesn't have any flower buds on it yet, and cut those leaves off right at ground level. Stick them in your handy plastic bag (aren't you happy you brought that along?) and head on home.
Once you get there thoroughly wash your dandelion, scrubbing behind the ears and removing all dead leaves, dirt and debris that may be left over from winter. Now pull off a leaf and give it a taste. If you are lucky, and picked a young dandelion that hasn't been exposed to too much warm weather, the leaves may carry hardly a tint of bitterness. If you aren't so lucky, you may find yourself making faces like someone who forgot the sugar in their lemonade. Either way you can eat your dandelion greens, but the bitter kind may take a bit of processing to be palatable.
If you have mild greens, with little bitterness, a simple salad will properly display the virtues of this famous potherb. Just toss the leaves in a little olive oil, lemon juice, salt and pepper. They go wonderfully with slices of pear and a bit of cheese.
For the bitter dandelion, you need a kettle of boiling water. Coarsely chop the dandelion leaves and place them in a pot. Pour boiling water over them, stir, let sit for a few seconds, then drain. Repeat two or three times with fresh boiling water. While the taste of spring dandelions may be greatly improved through this method, I don't know if boiling water is enough to rid dandelions that have already bloomed of their overpowering bitterness. If you have experience in this area, please let me know! Once you have reduced the bitterness of your dandelions to a palatable level you may then saute the greens with a crumbled piece of good bacon, or simply spritz them with olive oil and a dash of red wine vinegar and serve with a sprinkle of your favorite hard cheese.