Your best bet is buy an IBM Model M keyboard from Unicomp. That's where I bought mine. It is a really nice keyboard, solidly built, good travel on the keys. Yes, it's a mechanical buckling spring keyboard, so it does make noise. But my fingers just don't get tired. It's great. I ordered mine as a USB keyboard.
Unicomp also seems to sell a few keyboards in the more common "rubberdome" format, so maybe that's more your style.
I'll admit that sometimes my carpal tunnel flares up and I need to switch back to an ergonomic keyboard. I still have my Microsoft Natural Keyboard Elite from a few years back. It's a rubberdome keyboard, just like any keyboard you'll find, but it lets me keep my wrists and forearms in a relaxed position. It's a PS2 connector with a USB adapter.