It's not all the difficult to make a custom wireless keyboard. There are plenty DIY's depicting how to wire the switches, and just as many on how to program an MCU (mostly AVR), your biggest problem would be deciding what kind of wireless protocol you want. You can do Zigbee, Bluetooth, irda, hell even wifi with a serial to wifi-Ethernet adapter (wi-port). Especially with the onset of Arduino and it's plethora of iterations, it has made hardware creation much more accessible. All you need are some basic solder skills, a semi-decent iron, and patience.
So, find the wired mechanical keyboard you like (preferably one with a backing plate) and stuff it to the hilt.
Edit: but please, if you do use Arduino, I beg you, take the extra bit of time and learn proper C and how to use avrgcc. It will have much more benefit to you in the long run. It's one thing to use Arduino, its another to use it as a stepping stone to the real McCoy, that being the ability to take any AVR chip and extend your knowledge and abilities into new and untapped potential. Never stop learning.