Can i go to T-Mobile and buy an iPhone charger from there and not have a phone plan there?

Also if i can, can it be a third party company or just apple?

Yes you can just buy the charger without a plan or anything but it would be cheaper to buy from a 3rd party company. I would suggest buying from Amazon, I'll put the link to the Amazon brand iPhone chargers that are cheaper and trusted to work correctly.

Prior to iPhone 5
http://www.amazon.com/...004SBBD8U/

iPohne 5
http://www.amazon.com/...009SYZ8OC/

Yes you can buy charger without any plan or anythings. But you must buy original charger. Cheaper and 3rd party charger can't charge your phone 100% battery and charging time will so many longer than original charger. Ok good luck