Hough,
No one place will sell all it's K&N filters cheaper than everyone else,
certain ranges will always be more popular & thus attract a higher premium. it's best to find out exactly what make/model of K&N filter you need then do a shopping search. When you do search for the kit & get some cheap quotes take care because when searching for great deals, the cheapest supplier is not always the best. My advice is buy only from a company you've heard of even if it is a little more expensive.
Google shopper for example will throw up lots of great deals but a lot will be from internet traders you've never heard of & they may not even be in this country!
If you do find a good price for a K&N kit & you haven't heard of the dealer, put a post on here & see if anyone else has heard of them before committing you hard earned.