If you need to send keyboard-like commands to a computer, all you need is a USB-capable microcontroller. Those can be had for a few dollars from popular microcontroller series like PIC or AVR, or something like a Cypress EZ-USB. These microcontrollers can be had at Digi-Key and other distributors.
Loose pigtail USB cables can be had at Digi-Key and other distributors.
You could also get USB-capable experimenter boards and re-flash them to be HID input devices. For example, the Atmega8u/16u on the arduino Uno can be used like this, as can the Arduino Leonardo, and the clones of it (which are typically < $15, and perhaps less in bulk.)