you can get off the shelf 3.5" exhausts.
if you can weld you can make your own, i priced it up from an exhaust shop (this was for a cat back)
This is for a 3.5"
$95 for a donut (get 2 if you want to make front pipes as well)
$39 per M for straight pipe
$350 per custom dimension muffler, in my case 18*12*8 & 17*8*5. obviously an off the shelf size will be cheaper (eg 16*11*8)
$18 per flange (includes gasket)