Default Doubt about sim unlocking

Nooby question.Im going to buy on amazon a LG Optimus F6 locked to MetroPCS, but i want to unlocked it by imei to use it with another GSM provider outside the USA.

My question is, exists any problem for whom tries to get the unlock code if the phone has not the 90 days of paid service that carriers demand ?

Also, what site to buy unlock codes do you suggest me ?

Thanks in advance.