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Are there any kits out there other than the Forgeworld one?

Official? No. eBay, sure, just search there.

I'd say just buy a regular Knight kit and paint Chaos symbols and stuff on it, maybe attach a trophy or two. Raid the bits box for spikes and stuff. Here's one I did but never got around to finishing all of the paintwork on:

The FW chaos knight is supposed to be distinct from a renegade knight.

A renegade knight has turned traitor relatively recently where the chaos knight has turned long enough ago to start showing substantial physical changes.

I haven't checked the rules for 8th but in 7th renegade knights and chaos knights had different rules.

There's also a pretty decent aftermarket for 3D printed Knight components (not particularly cheap) with a bunch of alternate weapons, heads, etc. I'd assume there are some Chaos-friendly options out there.

Blood and Skulls also has a few alternate weapon options for the Knight kit.

The funny thing is, the FW Index has no entry for the pure Chaos Knight, they only have the Hellforged versions of the FW Knights.