Call customer service and tell them you're going overseas and you need some unlock codes.

Or you can buy them for cheap on eBay.

Did you know you can actually call the manufactor of the phone. example motorola :: you can call the 800 number for motorola customer service and if you tell them your make and model they will release the unlock code to you!

What phones are you trying to get unlocked? Some of them there are ways of getting around the codes if you "freak" the device. Smartphones for example can be unlocked very easily but dump phones are a bit more difficult.