The serial number can be found on the back of the installation guide or the CD case. You cannot get one from the internet to use pirated copies. If you bought the game like you should, and you didn't get a serial number, you're advised to return the game to the shop, or to call EA Customer support. You can find their telephone number in the installation guide (also of other EA games). If there's no phone number, go to www.ea.com
and find the customer/technical support section of your country's website. There you can find instructions for replacing a serial number, or you can use the "Ask a Question" feature (you might need to be viewing a FAQ entry for that) to contact EA tech support staff.
Note that serial numbers are not to be placed in this knowledge base, and asking for them is against the rules.
If you lost your serial number, see this page
for more help (US only), or this page
(UK and Ireland only, misprinted serial on manual) or this page
(UK and Ireland only, lost manual/serial number). You might also want to check the EA Games website in your language for "technical support" or "customer support" concerning the serial number, or for a phone-number. EA in your country can help you retrieve a new serial number. Go to www.ea.com
for the website of EA in your country.