Juston,
People have suggested plastisol transfers, which you would have to buy - you are not (probably) equipped to print them, as they are printed on screen printing equipment.
When you say "the heat press method" that includes any and every type of heat transfer out there - plastisol, inkjet light, inkjet dark, dye sublimation, chromoblast etc.
You should spend a few days and read these forums extensively. The answers you are seeking are there, but you do not yet know enough to ask the right questions. There are a lot of variables. There is no generic "best" only "best for this exact printing method on this exact type of shirt with this exact type of artwork." Until you specify those things, no one can really answer your question.
Not trying to dissuade you, trying to help you help yourself.
Good Luck!