This is where I'll post all the wonderful sites I get some of our workbox "fillers" from. I hope their as useful to you as they are to me!
Confessions of a homeschooler is where I get a lot of my daughter's workbox items from. She has great printables that my daughter enjoys.
Super Teacher Worksheets is a great place to great supplements for older kids. I love the reading comprehension pages, analogies, and math pages.
Craft Jr is a great place to get seasonal/themed mazes, mad libs, and word puzzles. They also have some great craft ideas and kids recipes.