A universal cataltyotc conveter might work to turn off your Plymouth Neon's check engine light... in otherwords it will get your Neon to emit cleaner exhaust, however it will not pass the California smog check if the CAT is not California approved; it will cause a "tamper" failure. With that said, you must replace with an OEM component.
The correct CAT will have an executive order (EO) number stamped on it indicating it is legal for California sale and use. As of January 1, 2009 it is no longer legal to import or sell 49-state emission legal catalytic converters in California; and that's why you will not find any online retailers shipping to CA.
The allowed option is to replace your Plymouth Neon's catalytic converter with an inexpensive (compared to dealer item) OEM CAT which is California approved and has been given an Executive Order number.