For sensible money theres only really open induction kits which stuggle to give gains due to the risks of taking in hot air from around the engine, but give a loud induction roar. Closed IK's like the pipercross viper and ITG maxogen are popular but more expensive and a little less noise.
The popular choice is to change to an aftermarket panel filter in your standard airbox and upgrade the cold air feeds.
A few (including myslef) have fitted the airbox from the older Espace V6 as used by the cup cars. But with all of them, there is very limited gains to be had