Manufacturers change their numbering schemes over the years, and the numbering schemes can be decoded. But where are you looking? Both Denso SRK20R11 and NGK IFR6A11 both are available from a number of vendors for immediate shipment.

As far as NGK plugs go, I believe the Iridium IX line was "top of the line" available in 2006. They now have one more iridium line, as well as a ruthenium line, that are "better" (at least in theory) than the IX line was. See: More Information for NGK 4589

 

Any good parts store will have a list of plugs that will fit your vehicle or as a last resort use a good shop.

 

I don't know if this information can really help. But if you're replacing spark plugs. Check RockAuto's website, I like to use Denso or NGK parts a lot.

I bought NGK Ruthenium plugs. I wanted to give it a try the reviews I read around seemed positive. I read a lot of things about counterfeit parts being sold on Amazon and eBay so I ended up price-matching with Advanced Auto Parts. I think it's been like 10 or 20K miles since I've had these installed and I don't have any complaints. Feel free to give it a shot if you feel comfortable trying these plugs. If you don't want to try these plugs I would just stick with whatever originally came with the car. Probably some sort of Denso plug. Best of luck though.

 

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Denso 3297 or NGK 4589 are the Iridium equivalents to the part numbers you listed, or get the Ruthenium ones (94279)

 

sparkplugs.com

 

Yep, yet another source. Specifically https://www.sparkplugs.com/application/search?specs=165401,165606

I can find Denso, NGK, and a number of other well known makers (e.g. Bosch, Autolite, Champion) iridium and ruthenium plugs at source after source after source for immediate shipment. I'm not brand loyal in spark plugs other than sticking with brands that have been around for a very long while. I do want platinum, iridium, or ruthenium electrodes and pre-gapped.

 

Autozone? Advance? Pep Boys? O'Rielly's?

 

Buy plugs and other crucial parts only from a Toyota dealer!!!

Genuine Toyota Parts and Accessories: Official Online Store | Autoparts.toyota.com
Enter your VIN, type spark plug in the search and press enter. On the left side menu, click 'hide unknown fitment' and then check the box. Everything that now appears is 'Exact fit' with a green check mark showing. Only purchase parts showing the green check mark.

 

Buy plugs and other crucial parts only from a Toyota dealer!!!

You are entitled to that opinion, and to express it, but I, and many others, do not share it.

Toyota has the parts it sells made by other manufacturers, and there's no "special secret sauce" applied. There are also higher quality parts than OEM, if one wishes to seek them out. Ruthenium electrode plugs being a perfect example.

 

NGK IFR6A11

Buy plugs at your local Toyota dealer. Looks like the Memorial Day 20% off on parts at Toyota is still alive at the checkout screen.

The price can't be beat, even when compared to places like Rockauto or Amazon. You don't have to pay for shipping and you can be certain the parts are genuine and not fake.

Spark Plug #90080-91184 | Autoparts.toyota.com

Boost your Toyota's performance and fuel efficiency with our genuine Spark Plug. Essential for optimal combustion, it ensures safe and reliable driving.

autoparts.toyotawestcovina.com

 

Denso or NKG. I wouldn't use anything else. Stick to what the OEM uses.