Engine Family# approves 2 catalytic system BUT I have 1 cat only

I have a 2003 Celica.
I have a check engine light came on about 2-3 months ago, the code is PO420.
The car runs fine, I haven't noticed any difference.
I need to get a smog check so I was gonna get the cat fixed (by welding) for about $300.
Than in the shop they saw that the Engine Family# 3TYV01.8FFB is approved for two cat system but I have one cat only. That means that this cat system I have is not approved in CA? How stupid is that! They can sell this car here but I can't get it fixed? There is no aftermarket cat converter available for this car.
I'm in CA and here I was told that I can only get the cat from a dealership which could cast me about $1,600! - that's insane!!!
To get a used cat is illegal.
I'm not sure if I have to replace the cat at all, a detailed diagnose cost about $120.
However, I've got a diagnose at Pep Boys for $45, but I'm not sure if it was accurate since they told me it is the cat AND the rear Oxygen Sensor and they usually don't go bad at the same time. It could be one or the other.
Any suggestion how to get it fixed with a reasonable price?