Yeah, kali is the obvious choice. It's effective and very easy to find.
Just as effective, but not so easy to find are the Western schools of knife fighting. German knifeplay is particularly good and fun to study.
There's also the Japanese stuff found in some koryu jujutsu ryuha, but you'd probably need to come to Japan for that, unless you can find a good ninpo school.
what styles teach knife fighting??? 
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