Either a Yamaha yz 400 or 426 filter is same you just have to carefully with a sharp razor blade cut the o-rings off old filter and glue them on new filter with super glue.I will confirm this tommorrow or you can just bring your old filter with you and compare.
Cannondale had stopped producing filters and ex employee Steve Maslanik is now the sole provider of Cannondale paper filters. I'm not sure if he is registered here but on Cannondaler he is MAZmx248. He plans to start a parts website soon but in the mean time he can be PMed and will get you setup.