A certified cable means it has been confirmed to work and meet the spec, but that doesn't mean that one that isn't certified won't work AFAIK. For 24p, I would imagine that most cables will work just fine.
http://www.bluejeanscable.com/store/hdmi-cables/hdmi-cable.htm. I see no mention of 24p on their site, so I don't see an issue. There's a lot of info on their site about what matters and what doesn't with respect to HDMI. I buy all my HDMI cables only from them.