I'm not sure how you are watching OP but they have dubbed past that. My brother just got collection 21 last week and it has english dub. That goes up through episode 516. Thriller Bark ends at episode 384 so if you've only watched up to the end of that saga you've got at least 132 dubbed episodes available yet.
I have heard that Funimation is stingy with it's dubs on streaming so maybe that's it? Look for the collections. You'd want to look for the collections 16 through 21 for the episodes you've missed. If you can afford any of them they're currently on sale on rightstufanime.com for half off. $18 a collection, each collection has 23 episodes.