Do this...
1. Call the best rated muffler shop in your area. Check Yelp, look for lots of strong reviews.
2. Call the shop and tell them you have a new RAM 1500 Hemi and the sound you want. They will recommend the mufflers they think will achieve that. Most shops will carry in stock or can easily order any muffler (or full exhaust system) you want for the same price if not cheaper than online. Most mufflers available are in the $90 - $110 range.
3. Arrange an appointment and have them install it. Depending on their labor cost, most will charge anywhere from $200 - $300 for the muffler, installation (welding, piping & hangers) and tax out the door.
Note - If you live in a CARB state, don't weld anything directly to the catalytic converter. It has a 7 year warranty by law and will be voided if you weld anything directly to the cat.