Man, the craziness I read on here sometimes... :rolleyes
OP, I'm pretty sure there's a cat that is listed as a 49 state replacement that's
also approved for use in CA on your car (most likely listed for the same or a different vehicle), as the aftermarket cats are, in virtually every case, manufactured to fit many different vehicle models with a single part number. It would be cost-prohibitive for an aftermarket manufacturer to produce a separate part for each and every car ever made that was equipped with a converter. If the same model listed for your 49 state car is also approved for use on a vehicle that is 50 state or CA approved, it will have an EO number, and that EO number will have a listing in the ARB database that states ALL the cars it is approved for, not just the 50 state or CA emissions cars. If this is the case, you're good to go.
<edit>
So I got ambitious and did a few minutes of research. :nerd
According to the EO I've attached here (D-182-48), you're looking for a Walker 81807. AutoZone lists this on their website for $105.99 each. Cross-checking the number to the application on their site verifies it is the correct one for your car.
I suggest you either look for a muffler shop that will install your customer-supplied parts (this shouldn't be a problem if you bring the EO with you), or find a shop that sells Walker cats and have them do the job. I think you'll be out the door for a lot less than a grand for both cats, either way you go. :cool
I don't blame you for wanting to keep the W210, they drive well and last a long time (well, except for trying to keep those huge headlight lenses from clouding up due to UV light from the sun!).
arty
Damn, the file is too big to upload here, so here's the link instead.
http://arb.ca.gov/msprog/aftermkt/devices/eo/D-182-48.pdf