Are there third-party mechanical keyboards that strictly have the Apple layout?
If you want "clicky" mechanical keyswitches, you should look at the Matias Tactile Pro, for US$150. It is a modern version modeled on Apple's long-discontinued mechanical-keyswitch keyboard that was sold in the 1990s, the Apple Extended Keyboard, which was before the days of USB.

The keyboard layout of the current generation of MacBooks and MacBook Airs is probably not 100% identical to that of the Matias Tactile Pro, but I believe this is as close as you are going to get.

Note that you can remap any functions of Function keys in Mac OS X in software, if anything doesn't match up to your liking.



Matias also makes a Mini Tactile Pro that saves space by doing away with the numeric keypad.



Das Keyboard Model S Professional for Mac, at US$133, is also highly rated, although the key layout in the numeric keypad and certain other keys is slightly different that what Apple uses today.