KegLand's 500ml Amber PET bottles come with reusable screw caps, but these replacements are available if you prefer to use new caps with each bottling run. The locking ring is left behind on the bottle neck the first time the cap is screwed off, so if you're using fresh caps you'll want to remove the locking ring from the previous cap.
These caps can also be used to replace the threaded caps included on the lid of the FermZilla fermenter.
KegLand Part Number: KL25881