What car is it?There are mixed opinions on upgrading the filter but I always do, I find that the car revs more happily.Cheapest place is ebay but make sure you get a branded one - K&N/Pipercross.
Also be very careful when it comes to servicing the filter. They need re-oiling and if over done it can mess up your MAF sensor if your car has one.
It's a mk5 golf gti, they dont seem to sell them on ebay for the gti, can i just get one for the golf as long as it covers the year of my car?