I haven't had to replace one of these, and they're truly ergonomic. I switched to an ergonomic keyboard after writing my dissertation gave me pain in my wrists using a standard keyboard. Getting used to an ergonomic keyboard makes a world of difference for wrist pain, and it's completely natural to switch back to a conventional keyboard. I'd also be careful in buying Microsoft ergonomic keyboards. These tend to separate the left- and right-hand keys, but do not slant the keys to match the natural angle of your hands when typing. The above Adesso keyboard (and keyboards from other manufacturers) have angled keys that more closely match the natural orientation of your hands when typing.
Another great input interface is the trackball. I use the Logitech M570 Wireless Trackball. I've found that keeping your mouse arm stationary goes a long way for wrist and arm pain too. These take a bit of getting used to, but they're well worth the commitment. The only drawback is that I have to clean my trackball once every couple of days.