KDE uses simple PNGs, stored in a quite easy to understand folder hierachy.
Icons are usually stored in /usr/share/icons/[ThemeFolder] or something like this.
They're spiltted 1st by size and 2nd by category.
Now the tricky thing is to get some nice PNGs...
There're a lot of great mac icon sources (hey, they home PhotoShop...) on the web, i certainly cannot link them all.
(But you may inform me if i terribly missed some important)
Good sources are usually:
Apple stores icons of all sizes in containers, suffixed ".icns" (some bad taste?).
This (proprietary?) format is not natively supported by KDE, so you'll have to do some conversion.
Luckily there's a linux application by Mathew Eis. that you can now get again from http://eisbox.net/software/
In the last time, (mac) icons are oftenly spreaded as icontainers, a format created by Pixadex, what is basically a conjunction of the contained .icns plus some meta info (that also differs between the original icontainer and the candybar icontainers...).
You'll need some pixadex product (mac-only) for full handling of those icontainers, but for simple extraction of the contained icons, i hacked together some short C code, you can download here
Notice:
This is a very first and rough attempt. It will extract icons from icontainers, but omit names, candybar IDs and copyright info and may create some (additional) invalid icns - read below for conversion process.
StuffIt is the traditional Mac compression aplication, like WinZip was on M$ Windows.
Linux users can use the *nix shell application unstuff to extract those .sit archives.
Now, how do you get the icons from out whatever you downloaded?
unstuff -m=auto -e=mac foo.sit[.hqx]
unstuff -m=auto -e=mac Foo.sit
incontainer2icns foo.icontainer
icns2png foo.icns
icns2png *.icns
for name in *.icns; do icns2png ""; done