When you say "transformer" do you mean an box containing an external power supply for the home theater or a step up or down transformer so you can operate the home theater on a different AC voltage than the unit is made for?
For the first you would need to contact the manufacturer or look to see if the unit has a marking to show what voltage, wattage and if it is AC or DC then look for a compatible power supply. A repair shop should be able to help if you can't figure it out.
For the second you just need to know the wattage that the HT draws which is usually marked on the unit. I would get one that is at least twice that number.