Are you looking for long tube or short tube headers? There are lots of brands out there...Kooks, JBA, Stainless Works, BBK...etc. Mine are from stainless works and I bought them from speedlogix for something like $1450 shipped (with high flow cats). The nice thing about SW headers is they come with 2.5, 2.75 and 3.0 inch adapters so you can plumb up to just about any catback size. Since you have a 2015 you will need the PCM to be sent in to be modified $250, a tuner ~$600 (depends on whether you go with HP tuner or Diablo interface), and a tune ~$400 (email tunes are cheaper than dyno tunes but dyno should give better results). As for power gains this is one data point.
So I’ve been looking into getting long tube headers for my RT shaker. I’ve been doing my best to do my own research through different threads. But I’m at a dead end. What is needed to install long tubes on my 5.7? I called the muffler shop and they claim that I can only do shorties. Is that true?
Shorties won't really change sound. Long tubes will give some sound and power but then you run into emissions problems in some states. Ultimately if you want it to be louder delete the cats but even worse emissions problems and the smell. Think more about long term goals with the car before deciding on headers and come back and we can give some better advice