Vit, my 54 plate has an original back box the "clam shell type". If that's what you are after Euro carparts sell them. Ask young Andy Wii as he fitted one on his car and they are the spitting image.
I know the factory units are all in one length but when I went to my local exhaust fitters they quoted £88 for a generic one that looks nothing like the original. After 20 mins explaining to them they quoted £118 for the clam type.
Hope this helps.
