If this were me the first thing i would do is take it to another garage,they seem incompetent

I do'nt suppose the exhaust on your car was an aftermarket full system ?.Maybe it was made as a complete system so a standard box will no longer fit.
They come up on Ebay very often but be aware of whats actually on offer.I have seen a few where they say stainless but what they really have is only stainless boxes with mild steel piping.Stainless steel comes in various grades,good stainless will not attract a magnet at all,cheaper stuff will.
I got my Powerflow cat back system for £160 from a guy about 5 miles away.He paid just under £500 for it less than 6 months earlier (have receipts).Took car in for mot and inner sills were gone big time which coincided with a Cosworth Sierra he wanted so he just broke the car up and sold the parts.
Ya pays ya money..................