Cherry MX switches: http://cherryswitches.com/us/product/keymodule-mx-series/
A few other keyswitch manufacturers listed here: http://www.mechanicalkeyboards.com/mechanical_switches.php
Available at Digikey and other places for slightly more than $1.00 per switch.
Ebay has lots of different cool sets of double-shot keycaps for around $40: http://www.ebay.com/itm/Double-Shot-Keycap-104-Set-for-Chery-MX-Keyboard-Key-Caps-Red-White-/281434960552?pt=LH_DefaultDomain_0&hash=item4186d4b2a8
Teensy works as a USB keyboard controller: http://pjrc.com/store/teensy31.html
Info about scanning matrix of keys, diodes, ghosting, and masking: http://www.dribin.org/dave/keyboard/one_html/
USB keyboard info: https://en.wikipedia.org/wiki/USB_human_interface_device_class
An open source hardware split keyboard: http://www.key64.org/
Similar or related project with BOM: http://www.ergodox.org/Hardware.aspx, uses MCP23018 16-bit SPI I/O expander
Geekhack keyboard zealotry forum: https://geekhack.org/index.php
Mouse: Dave Mellis built an OSHW mouse, but the sensor is no longer available: http://web.media.mit.edu/~mellis/mouse/