Hi mate and welcome.
First thing is do a search as most newbee questions have been asked a million times before.
You can buy a 'performance ecu' from anywhere. What you first need to decide is what do you want it for, how much you want to spend, has your tunner any experience in mapping euc's?
You can buy a GReedy E-Manager for a few hundered quid while a Motec will cost you £2,500!!
Use the search function and have a read!