Most of us carry around a flatpick or two. I watch folks fish for theirs, pulling out keys and change, and hunting through both pockets.
You can buy pick holders and dispensers at music stores. They're like coin dispensers, and always seem too bulky to be practical, though I suppose you could keep them in your case. When I see them, visions of toll-booths for musicians flash through my head.
The coin analogy is useful, though, because coins are roughly the same size and shape as picks, so solutions for one work for the other.
Coin pockets are my first choice. I'd bet I always have a pick in the coin pocket of my jeans.
As an aside, does anyone keep coins there? Ever? They seem like natural homes for picks, and a real use for an otherwise-pointless sartorial feature. I try to remember to take the pick out before I toss the jeans in the wash, but I'm sure I'll be retrieving flatpicks from the bottom of the drier when I'm 80.
For more than one, though, that pocket's too narrow. Stick in four or five picks and you can't get them out easily. Plus, you have to pull them all out to find the one you want.
Here, the right tool for the job is a coin purse. They're flat, don't add much bulk, and open wide to let you fish out just the coin you want. Works for picks, too. I carry one in my trousers, keep another in my case.
I particularly like the squeezable, leather, spiral-opening, "rose" purses. They're the price of a couple of cups of coffee and I get complements on them. That translates to "cheaper than plastic and better looking." Right now, I order mine from Amazon -- no muss, no fuss -- but I should remember to suggest to the guys at my local music store that they stock them, too, right next to the picks.