after-market catalytic converters for 2004?

I assume your car has DTC P0420. Since you are talking about aftermarket I assume you plan to DIY.

In that case, I suggest that you first replace the air/fuel ratio sensor which is upstream from the catalytic converter. If that is marginal, that might cause the DTC to appear. It's much cheaper and easier to replace that sensor than to replace the catalytic converter.

Then, if you still need to replace the catalytic converter you can buy the correct one for $1,106 plus shipping here:
Champion ToyotaWorld

Good luck.

 

You would have to separately order gaskets and sensors. The sensors don't have to be replaced.

The reason I suggested you replace the air/fuel ratio sensor first is because the engine ECU expects the sensor upstream from the catalytic converter to show rapid fluctuations in oxygen level, as the air/fuel mixture is varied by the engine ECU; while the sensor downstream should show fairly steady oxygen levels, which proves the catalytic converter is working.

If the air/fuel ratio sensor is worn then it will show slower fluctuations in oxygen level, and the engine ECU may erroneously conclude the catalytic converter is bad.

 

If you're set on going aftermarket, TurboTrix in Edison will probably install the cat for you with a minimum of hesitation.

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Rock Auto has them for $264.79. You can get any muffler shop to install it.
2004 TOYOTA PRIUS 1.5L 1497cc L4 ELECTRIC/GAS FI [1NZFXE] : Exhaust : Catalytic ConverterPriceCoreTotalWALKER Part # 16337 {***No Longer Legal for Use in California***; Direct Fit}
If Welded Assembly, Replace All Required Parts; Fed. & Calif. Emiss. Models. Not legal for use on OBDII vehicles licensed in the state of California
$264.79$0.00$264.79

 

I never changed it myself on a Prius. I have done exhaust systems on other cars. It is generally a pain to work under a car on the exhaust, which is why I suggested going to a muffler shop. You should pay 2x the rock auto price installed at most. I would take a good look at the exhaust to see if it will come apart, or if the resonator would need to be replaced also. Here is the resonator:
004 TOYOTA PRIUS 1.5L 1497cc L4 ELECTRIC/GAS FI [1NZFXE] : Exhaust : ResonatorPriceCoreTotalWALKER Part # 55431 {Assembly}
If Welded Assembly, Replace All Required Parts
$81.79$0.00$81.79The ends of the resonator and catalytic converters have flanges, so they should be easier to remove at those points. You will need new gaskets and bolts, so check if they come with it, or you need to order them separately.
As Patrick said, it may be wise to try the air fuel sensor first. I don't know how long they last, but Walker is a good manufacturer. The sensor is fairly expensive too:
2004 TOYOTA PRIUS 1.5L 1497cc L4 ELECTRIC/GAS FI [1NZFXE] : Fuel/Air : Air/Fuel Ratio SensorPriceCoreTotalBOSCH Part # 15624 {Wideband A/F Sensor - OE Type - Exact Fit}
Upstream; 4 Wire; Wideband A/F; Check / Replace Interval: 100,000 Miles* Non-stock item--shipping delayed up to 4 business days
$165.79$0.00$165.79
Good luck!

 

1. I've only had to replace the catalytic converter on my BMW 325i convertible many years ago, and used an aftermarket unit which was approved for CA sale.

2. Based upon the photos that vertex provided, it looks like you'll have to purchase the resonator as well. This replaces the Toyota part which is one piece. Who knows how well this will fit into the available space. This probably is not a DIY exercise unless you are equipped to do bending of the tubes (and maybe some welding).

3. Yes, you would have to acquire gaskets for sure. It's up to you whether you replace the air/fuel ratio sensor upstream and the oxygen sensor downstream from the catalytic converter.

5. Hopefully a new catalytic converter will work well enough to turn off the check engine light. It probably won't last more than 3-4 years, this is why that part is not approved for use in CA.