My son has given me a Canon Pixma MP 780 all in one but there is no user manual or setup CD Rom and no usb cable. where can I obtain these items please?

1) Printer USB (A->B) cable - this can be purchased almost anywhere, where computer accessories are being sold. If you want a cable longer than 5 feet (1.5m) then go for a better quality, thicker cable.
2) Printer manual and drivers/software - you can download them from Canon support website. Go to this address:
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDetailTabAct&fcategoryid=334&modelid=10444
choose your operating system from the drop-down list on the right and you will be presented with available downloads.
For starters, I would recommend downloading the manuals (they are far towards the bottom of the page). When you get them, you will know better what software you want.

For basic printer operation you will need to download and install the printer driver (and updater if present).
For scanning and additional functions you will have to download also the Easy PhotoPrint application, perhaps others.

Please note: first install the driver, and then plug in the USB cable - most probably you will be prompted to do so during installation.

Good luck and please come back with a testimonial if helped :)

Go to http://software.canon-europe.com/ and select you type of printer
then select you computer operating system and language.
on the bottom of the page you can download the full manual