I homeschool my 8 year old boy and my 4 year old girl. They are both advanced. I had alot of trouble finding secular curriculums for my children. We are also on a very tight budget. The first thing I did was go to my states public school website. from there I downloaded The Grade standards. This way I could make sure that my curriculum choices match my states standards. Then if i chose to send my children back to public school they would fit right in. I live in North Dakota. We are one of the toughest homeschooling states. Here is the list of books I use for my son:
Vocabulary and Spelling---Wordly Wise 3000
Science--- I use the older versions of Harcourt.(This way I can buy them on Ebay for cheap) Brown Bag Science and Evan Moore Scienceworks.
Latin---Latin's Not So Tough
Poetry---Evan Moore Read and Understand Poetry
History---American Education Publishing The Complete Book of United States History.(This is a grade 3-5 book, I don't know if they make one for older grades)
Handwriting---Handwriting Without Tears. (If your kids still need handwriting practice)
Spectrum workbooks are also great. They are aligned to most state standards. They are also cheap ($7-$10 per book).
I hope this helps you.