There are ways to do it like buying unlock codes online but you are throwing your money away. Not to mention your financial info. Also not legal.
Can I ask why you don't want to contact the carrier?
Welcome to Android Central! It's still worth contacting the carrier that the phone is locked to, and explain to them the situation. They might help, or they might let you know if the phone is blacklisted (i.e., the previous owner refused to pay off the phone, which is tantamount to stealing it, and therefore the IMEI gets blacklisted so that it can no longer be used in the US).