Yeah ebay. Do your searches under Zoids parts or Robostrux parts
Mr. Furious: D-do you mean my secret identity? 'Cause I couldn't...
Monica: No, I just mean your name.
Mr. Furious: My name. Ummm... wow. Okay... It's... Phoenix... Phoenix Dark... Dirk... Phoenix... Dark Dirk. I was christened Dirk Steel and then I changed it to Phoenix...
Monica: Forget about it. It's okay.
Mr. Furious: It's Roy! That's, that's my name. My... my real name is Roy.
Monica: Just. Be. Roy. Okay?
I never understood why Tomy never decided to sell replacement caps, as there is quite a lot of demand for them. The company still re-issues old Zoids models, so it's not like they lost the molds or anything.
As an alternative, I've seen some people fall back on rubber tubes that are used for aquarium filters to replace bristle Zoids caps. You buy the appropriate diameter tube, cut it up into cap-sized format and then put it on your Zoids. It doesn't look the same, but it will keep your Zoids together and enable you to make them functional again.