If you are buying from Amazon that means you're buying the Viz version, since the Chuang Yi version isn't there. But volumes 15-onward aren't there yet because they only put up new volumes every so often there since Viz is still working on translating them. Give them a bit of time and then you should start seeing the other volumes, but you'll probably just see one or two new ones every so often. Just so you know volumes 15-29 will be under the same title, so just look for them like you already are.
And uh, be sure to look for Pokemon Adventures: Diamond, Pearl and Platinum volumes too. They are volumes 30-38 in the Japanese release, but Viz is releasing them as a separate but related series called 'Pokemon Adventures: Diamond, Pearl and Platinum'. Right now only volume '1' of this separate series is out, and volume 2 will come out on June. So be sure to look for those too, by searching Pokemon Adventures: Diamond, Pearl and Platinum volume 1, 2, ect.
But if you don't want to wait on Viz for volumes 15-onward, then I recommend doing what everyone else is doing, and searching for volumes 15-onward of Chuang Yi's release on sites like eBay and what not. They have up to volume 35 out right now. You don't have to if you don't want to, but do keep in mind if you read Viz's release they won't catch up 'til 2014, and by then Japan will have even more volumes out.
"But sometimes I have to ask myself this question. It's true that to us his imaginings are nothing but the inventions of a busy mind. But to him, there simply is no other reality. Furthermore he is happy there.
So why, I ask myself, why in the name of healing him must we drag him painfully into the world of our own reality?"
~Doctor's memo, from Silent Hill 2