I know that Platinum and Cupper are recommended.
How about Iridium?
Anybody.....?

Used Iridium in many different cars, including a 1st gen 99 I30 and a 90 M30 and saw no difference compared to the original NGK platinum plugs the cars came with. The only difference I would expect is better longevity of the Iridiums. However, I usually wind up changing platinum plugs less than half their recommended lifetime since regardless of the material it's made from, a fouled Iridium plug will perform the same any fouled plug.

Copper plugs provide better conductivity, but don't last as long as platinums. I'd suggest getting the best of what you can afford that's as good as or better than the original I30 NGK's taking into consideration how often you'd like to change spark plugs.

I am on iridium now, but will swap out for Platnum when I get the chance. They are in there for the old nitrous setup, but I don't run that. It runs fine on them, though.

NGK copper here. I fiddle around so much with the motor, changing plugs every 20K is no big deal.

yep, our plugs are so easy to change your crazy not to use coppers.

Any website where we can buy NGK spark plugs .. specs ?? Or are they available in Napa or Pepboys

I would not go that far. Every week we have a post about problems changing or removing plugs...plus swapping the out always runs the risk of damage to the coil packs. Sure, it is not commone, btu it happens to people on here all of the time.

I got mine on eBay for $40 shipped for a set of 6 Platinums. I have Iridiums in the I30 b/c the previous owner ran N20, but eBay is a great place to get the NGK platinum ones for under $50 w/ shipping.