There are two options. Stock or Aftermarket.
1. Stock one can be got at your local suzuki dealer. It is a paper filter that needs to be replaced ever so often.
Srinath has sucessfully used gasoline to clean this airfilter by soaking it in gasoline and letting it airdry. If your cheap this might be a viable solution.
2. Aftermarket has two options. The most popular one is the K&N Lunchbox / Clamp on filter, which requires the removal of the stock airbox. Its model # is RU-2970.
http://www.usdieselparts.com/knfilters/ is a K&N authorized dealer and has that filter for about 33$ + S&H.
There is also a K&N Drop in filter that goes into the stock airbox. This has the option for a restrictor that can be removed.
Model #: SU-5589
Kerry's K&N Drop In Filter Page (http://www.bbburma.net/K&N_AirFilter.htm)
Caution: Using an Aftermarket K&N or UNI filter will most likely require you to Rejet. Bikes come jetted really lean and would require additional parts to get your motorcycle running right.
Links:
Also See:
Rejetting your bike (year)
Removing your Airbox.