That's S&W proprietary grip. They'll sell you one. I think they have a 800 number and a web site. Brownells and Midway might have them, too. That I'm not 100% sure on, but they have most S&W parts (when they can keep them in stock).
Pretty much all standard production revolvers have a round butt, these days. The M629 in the picture you linked to has what's called a round-to-square conversion grip. Those will only fit a gun with a round butt frame. So, you need to be sure that's what you have.
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~ S&W aficionado in training ~
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