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.

Honestly the only "real" choice I've come across that works the "best" is the *Matias mac specific keyboards

I have experience using both the Matias Laptop Pro and the Das Keyboard 4 on mac and I think that there is a subtle reason why Matias is best. I'm sure this also extends ot their other keyboards.

X & Command

As you can see here is the Bluetooth Apple keyboard layout - notice how the X and Command buttons are aligned

On the Das keyboard you can see that "Alt" (which you would map to command in "mac" mode) its not aligned very well to the X button

And lastly we have the Laptop Pro where X is almost perfectly aligned with the command button

So What?

So why is does this matter? A majority of the mac shortcuts involve the command key and I've noticed that having a shorter travel distance for the left thumb is better. When I use the Das I find it more awkward to use mac because I'm always having to use Alt

Answer

So my answer is any of the Matias Mac keyboards are really your best bet. If you don't' use keyboard shortcuts then this isnt' as much of an issue.

I personally use two that do and have been please with both of them. I'll briefly describe each below:

WASD CODE Mechanical Keyboard

WASD makes many keyboards and can customize them to your specifications (layout, key color, mechanical switch type, mac/linux/windows, etc.). The CODE keyboard is especially nice and has a clean layout, white backlighting and neutral font (Helvetica). It's made with two types of switches (Cherry MX Clear and Cherry MX Green with rubber o-ring dampeners). I personally use the CODE 87-Key - Cherry MX Green (they also make a 104-Key version, if that's your style.) While the Greens require more force than most other keyboards, I've found it inconsequential, partially because of the dampeners.

Since you mentioned having a Mac specific layout, the CODE keyboard has a DIP switch on the back of the keyboard that allows you to set the keys at the hard wear layout. Also note that the command (⌘) key is blank but can be replaced with a Mac specific version, if you so desire, and can easily be moved to the usual Mac layout—a key puller is included with the keyboard.

There are many other advantages to this keyboard, including n-key rollover, so definitely have a look at their website.

Razer Blackwidow Ultimate

I also have the 2013 Razer Blackwidow Ultimate (2014 version), which as far as I can tell is only sold on Razer's website. It uses Cherry MX Blue keyboard (great for typists) and has a very solid build to it—e.g., key backlighting, braided cables, usb, microphone, and audio ports, and a solid frame. However, while the hardware has so far been solid and I haven't had any problems with the keyboard itself, the included software (Razer Synapse v2.0, which manages macros and macro profiles) has proven quite buggy and as far as I'm concerned, worthless. Thankfully, you can forgo the software and simply use the keyboard as is.

Das Keyboard

I tried the mac version but was immediately put off by the glossy surface. Too each their own...

Matias Quiet Pro

I also tried out the Matias Quiet Pro. It has a great feel to it and is relatively quiet, if that's something you're interested in. While it is built well, I didn't like the plastic that formed the base: it looks cheap to me. Additionally, the frame wasn't quite as solid as the WASD and Razer keyboards previously mentioned.

Recommendation

If I had to buy again, I would not hesitate to buy one of WASD's keyboards. They're absolutely solid.

Alternatives

There's a great site that has a large collection of mechanical keyboards you may peruse. Have a look at MechanicalKeyboards.com.

I seriously doubt you'll find an 'exact' match for the Mac Keyboard w/mechanical keys and that has the same exact dimensions in terms of keyboard spacing. Low height keys and mechanical switches might prove difficult to come by.

I am seriously considering the DAS Keyboard Model S Professional for Mac. It has quality mechanical switches (MX Blues, a touch-typist favorite), and Mac specific keys (OPT, CMD), as well as function keys notation for volume, brightness, media, etc, and includes a SLEEP shortcut key.