If you are just looking to have them welded on, any exhaust shop that deals in Flowmaster should be able to order some in and weld them on for you...I think the IRS catback on the 99+ cobra's just has a 2 1/4"-2 1/2" offset/aligned inlet and outlet on the muffler...
Summit sells them too...I believe the part numbers are FLO-842444 for the 2 1/4" and FLO-842544 for the 2 1/2"...