have a look here
Frontier Parts (D40) 2005-2009 :: CourtesyParts.com
I bought my K&N from here and it was something like $30 and as far as I know only the KingCabs have different gear but all CrewCabs are same as far as I know
K&N Hi-Flow Flat Panel Air Filter
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Your Price: $44.95
In Stock: Normally ships out within 1 business day.
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K&N Hi-Flow Flat Panel Air Filter
K&N
Quick Product Details
* Manufacturer: K&N
* Category: Performance
* Product SKU: A60-KNFLT
Product Description
K&N uses premium quality oiled cotton gauze sandwiched between layers of wire screen.
This special design permits very high flow rates, improves throttle response and very long filter life, increases air flow.
When dirty, simply wash, re-oil and re-install (usually every 50,000 miles, see Cleaner Kit below).
Same size and shape as the factory OE air filter, slips right in.
Emissions legal in all 50 US states, works with Original Equipment manufacturer vehicle electronics and will NOT void factory warranty.
* Application:2004 - 2009 Armada 5.6 liter DOHC V8
* 2005 - 2009 Pathfinder 4.0 liter VQ40DE V-6
* 2004 - 2009 Titan 5.6 liter DOHC V8
* 2005 - 2009 Frontier 4.0 liter VQ40DE V-6
* 2005 - 2009 Xterra 4.0 liter VQ40DE V-6