Follow this link, it will take you to a website that deals with unlocking phone codes and it is also where you can order SIM's ahead of your trip.
http://www.moremobile.co.uk/unlocking-mobile-phones/
also you can go to this site for SIM's
http://www.telestial.com
Many phones can ONLY be unlocked via the cellular provider through the use of a special software and computer interface that the phone must be connected to. It is actually against the law for your phone NOT to be unlocked if you so request it of your service provider. I would go back to the store where you purchased the phone and demand they unlock the phone for oversea use. Do make sure that your model of phone will support the CDMA/GSM frequency of the country of travel. Many phones that claim to work overseas unless they are Worldband Phones, you may experience problems in certain countries. Do you homework first as I always say.
Have a great trip!