Speaking of which, I'd so purchase an original anime cell of my favorite characters!~ .з. If I ever see it aviable!~
One Piece is completely colored by computer, except for the OVA and the 1st movie (these are pretty expensive). So you'd probably have to settle for a production sketch. Sometime in the past, Toei started using computers to do their inbetween work, so you'll only see inbetween drawings (douga) for the earlier episodes and video games. The later episodes only have key drawings (genga) and layouts, settei, and storyboards in actual physical form. Plus Toei doesn't release any of its production art, so most of what actually comes on the market was probably smuggled out of the studio.
So, for such a long series, there really isn't much available. At least, there's not much available compared to some of the old, popular, cel-produced series like Cardcaptor Sakura, Sailor Moon, or Dragonball. You've got a good chance of seeing something new from those shows every day, and at least you'll see something old. One Piece art only really comes available once every couple of months, and in big lots, so if you only want one sheet or something, you might not win against those bidding to keep the whole set (they'll probably bid higher).
Oh, but One Piece does have reproduction cels! Those are basically cels painted after the show aired. They only pick a couple of shots, and I believe that they paint five of each shot. They don't have the one-of-a-kind specialness that a production cel has, but they're the closet thing you're going to get.
Who exactly are you looking to get? I have some key sketches from Thriller Bark for sale, plus I got a bunch of layouts from the episode where Camie is introduced, and I'm considering selling some of them. I haven't made up my mind yet. They're all cute, but I just have so many. :unsure: