Can I get a new muffler?
 

I had a new exhaust system installed about a year ago, but I want to get a new muffler. They charged me about 300 dollars for everything, if I buy the muffler online and have them install it will it be cheaper since I already have the dual pipes and everything?

It should be cheaper, but it depends on how it needs to be mounted. Some require welding and some require clamps. If it's just clamps, it will be cheaper, but yes, you should be able to replace the muffler.