Those factory air filers get dirty fast, and they are expensive at around $45 a pop. Here are some air filter resources:
OEM Filter (we recommend)
These filters are also available, but not recommended due to washable filter not having as much particle filtration as the OEM disposable paper filter. Use at your own engines risk!
K&N Washable Replacement Factory Filter
There is also the K&N Performance Air Intake System
KWT X2 (we recommend)
UNI Pre-filter UFI-UP-6300AST (economical option) **When using this filter, remove the red outer sock, it is not needed.**
S&B Particle Separator (works, but noisy and for the money and installation, we feel the KWT is a better option overall)
10-80 Dirt Sports Intake Kit replaces the factory intake plenum and allows easy mount of a prefilter on top of the fender. They have a full kit with filter, or an intake only kit if you want to use your own filter or rig a Donaldson Pre-cleaner on top of it (additional tube would be required).