You can easily get chains that high with weak monsters like wolf-types (Wolves and Hyenas) between Giza Plains and Dalmasca Sands. The strongest monster I could get a chain so far like that is the Werewolves in Giza Plains. You can run two screens west past the save crystal and back again, they'll reappear.

But there's little point in getting a chain of 999; you'd max your inventory of Loot from that monster type long before that. Maybe you could max rare drops that way.