I'm not sure where you would be able to purchase a copy of your user's manual, but you can download a PDF copy of it here on Casio's website:
http://bit.ly/aACFCk. You need to click the agree button, and a list of musical instrument manuals will be displayed (including the manual for this keyboard).
If you do need to purchase a physical copy I would suggest contacting Casio directly (they might just send you one for free).
Hope this helps!