The 1999-2000 copyright English Fossil set consists of basically all the unlimited non-holos (except the Zapdos holo, which is the only holo that can be found with the 1999-2000 copyright) in Fossil—some were released as part of theme decks, but others could only be found in red-logo Fossil packs from Australia (if I remember right). They can be somewhat difficult to track down and are more expensive as a result of their scarcity. Here are some threads discussing it: here and here.
And code variants exist in some of the EX era sets for common and uncommon cards. The code can be found in the bottom-left corner:
EX Delta Species, for example, has three code variants for each of the common rarity Ditto cards (Charmander, Squirtle, Pikachu, Geodude). See this thread (or the Google sheet referenced in the beginning). I think Ditto just has code variants for EX Delta Species, since the FRLG Ditto is rare (meaning only one code). Happy hunting!