Any Alternatives to OEM Catalytic Converter for TL?

I am a visitor to this group and not that familiar with Acuras. My stepson's 1999 TL is in need of a new catalytic converter. Most shops
will only replace "exact fit" converters of which I am told there are only two types, both pretty expensive. Has anyone had experience with having a generic converter installed by shops that specialize in custom exhaust systems? Any comments would be appreciated.
R. Frist
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