Can I get the chip or electronic kit of a USB keyboard alone whithout the plastic case?
All USB keyboards work by means of a matrix of keys.  There is no 1:1 relationship between a pin and a keyboard function.  If you examine the membrane you can see how the matrix is made up of columns and rows.  Rows and columns are scanned in sequence to find where they intersect at a pressed key.

You will find that (pretty much) all modern USB keyboards use surface mount components and ICs.  These can be pretty hard to work with, especially if they are soldered directly on to a flexible PCB.

Ideally you would be looking for an old USB keyboard.  One that has buttons that no longer work - ideally it would have been a high quality one in its day. Something quite a few years old - one of the first USB keyboards to be on sale.  They are less likely to be using SMD and more likely to have a real PCB inside you can work with.  Even if they are SMD they will still have a better chance of having a real PCB.

If you're looking for a chip to completely roll your own, the Alcor Micro AU9410 is a good choice, but alas is (like most similar products) only available as a surface mount device.

A third option is to create your own from scratch.  You would need a microcontroller with USB capability (say a PIC18F4455), and some firmware to emulate a "Human Interface Device" (or HID).  Microchip have examples in their application library.  You can then get that to do absolutely anything you want with the keyboard.