Yes sure You can buy GSM Phones like AT&T ,T-mobile locked phones ,Because they are easy to unlock it from Network lock,You cannot unlock CDMA Network phones like Boost,Cricket and Verizon etc...also after unlock it you cannot use with that network,You can get the unlock code from here
Ihav.net and unlock it from Network lock,then you can use with any GSM Network Sim card like Vodafone,Orange etc...Boost is CDMA Network So you cannot use with that Network Sim card.