As far as I'm aware of Eclipse's only have (1) Catalytic Converter anything else that resembles it would be a resonator (dampens the sound of the exhaust system).

Are you throwing a check engine light? I don't see how its a bad catalytic converter, its possible but I don't see how. If your check engine light is on I would go to Discount or somewhere that has a obdII scanner and check what the CEL code is. If its a bad O2 sensor then thats better then it being the catalytic converter.

If you truly need a catalytic converter go here...
Club3G

That's a website for the 3g Eclipse in which case your 2003 is one. I'm sure somewhere on that website/message someone is selling a catalytic converter or knows of a place to get one new or used. There's also a few locals on there that may be some help.

I've heard of dealerships asking for that much on catalytic converters, your best bet would also be calling a bunch of places to see if they have them or can source one out for you. Did you try calling some autopart stores? It wouldn't hurt to try.

Just trying to help. Hope atleast one thing I mentioned helps you.