Dr. Gas (
www.drgas.net) sells 4 inch oval tubing, and ive seen a spintech muffler that was 5 inch oval... but i havent seen 5 inch oval tubing... especialy not stainless
you might have to make it yourself, 2.5-3 inch round, cut in half lengthwise and joined with a 2-2.5 inch wide strip... (lots of welding)