Cheap center caps can be found at www.Hubcaps.com .
Try upstreamcentercaps.com if you want custom center caps. Otherwise you can find OEM center caps on ebay for a good price.
You can find affordable wheel center caps on hubcaps-wheelcovers.com.
Wheel intovations is a great place to find wheel center caps. They they are very friendly and helpful and there is a large selection of caps to chose from.
Generally you will be able to find replacement center caps at the dealership. You can also pick them up at performance shops like www.summitracing.com.
You can find a very wide range of center caps at hubcapscanada.com. They have many makes and models for any kind of hub cap you need center caps for. You can visit their store at 5905 Gateway Blvd. Edmonton Alberta. Or phone them toll free.
Pueblo craigslist/autoparts.
One can find information about cadillac services online by visiting a website made just for cadillac services. They provide a cadillac help center where you can ask questions about services.
The first place I would check is at your local junk yards. Most likely they will have them. If you can not find them there you can try hubcap heaven. They have just about everything for your rims there. If they don't you can try Amazon.com and they should be able to find some aftermarket center caps for you.
Car lovers can find center caps for their Chrome Rims even if they are old. You can search websites like Ebay or Craiglist to see if anyone is selling spare parts for center caps or reach out to a dealer and ask for help.
I found this one particular site most useful among others for information about all types and brands of wheel center caps- http://www.hubcaps.org/freeadvice.html AND http://www.hubcaps.org/
I am assuming that you want to know where the Catalytic converter is in your 1990 Cadillac. it is underneath the car, silver color (or rust) welded to the exhaust pipe about the center of the front seat floor.