AJ USA has some nice prices on K&N products, with free shipping on orders of $18.97 or more.
i ordered a K&N RX-4730: Universal X-Stream Clamp-on as a replacement filter for my AEM CAI. It took some trimming of an inner ring to get enough of a lip to properly stay on the piping.
Oh, and one word about recharge kits: the cleaner is very caustic, it tends to eat away at the pump of the bottle it comes in after a few months (causing it to work rather crappily), or other bottles you may put it in. Because of this, I just buy the cheaper, smaller size.
The site I've found the most competitive K&N pricing so far is at www.expressautoparts.com, if your order is going to be $50 or more to get free shipping. Their site is terrible to navigate through, so if you're looking to buy a K&N product (or most products), you'll need to know the part#. So if you're set on particular K&N filter, take the part# and plug it into their part# search at http://www.expressautoparts.com/accessories/INDEXPART.CFM. Don't enter the dash (-) in the K&N part#.