Sorry, there's no such thing as an unlock code for a particular type of phone. Every phone is allocated a unique 'International Mobile Equipment Identifier' (IMEI) and a specific code is then given to that IMEI. (i.e. if you had lots of Nokia 3510i phones, they'd all have different unlock codes because they've all got different IMEIs).
You can find the IMEI for your phone by entering *#06# on the keypad. You then need to provide that number to one of the many firms (online, or at your local market) which charge for unlocking phones. Alternatively, Google 'calculate nokia unlock code' and you'll find plenty of sites advertising free software which they claim will do the job for you. (As always with downloads, especially free ones, run a virus scan and ensure that you've got anti-spyware software on your PC).
Chris