Magnex and Janspeed are among the quietist sports exhausts on the market.
Magnex do make universal back boxes in their most popular sizes ie 3.5' round, 6x4 oval, twin slash cut and motorbike style cans for car fitment.
Ive only ever seen these on offer from one place and thats
www.demon-tweeks.co.uk not the cheapest but they do hold a bigger stock than most. I think they are around the £100 plus mark then any decent custom exhaust shop or good mechanic should be able to fit a custom back box and fabricate some new hangers etc.
hope that helps