can i buy a phone from t-mobile and get it activated from metro pcs?

No you can't use sidekick mobile with metro pcs sim until you unlock it from t-mobile network provider .You can easily unlock your phone through online providers like OnlineGSMUnlock.com here they are rendering network unlock code for all mobile models at reliable cost ...You first check that whether the code is available for your mobile model .If yes then pay for it and unlock your phone then freely use it with any GSM sim ...