There are public charter schools that uses K12 curriculum. You should look around before you buy it. In my state, California Virtual Academy uses K12. I have never tried K12, so I cannot tell you much about it. Instead, I joined a public charter school in my area. They gave me $400 a year to pay for books and they have free classes on campus a couple days a week.
We are using Michael Clay Thomson (Language Arts), literature and writing packets provided from school, Time4Learning (use when he is bored, just as a supplement to make sure he meets the state requirement), Zaner Bloser Handwriting, Singapore math, Story of the World (History) plus classes from school, Science 2 U (Classes for homeschoolers), Beginner's Geography and Mapping Activities (By Goematters), foreign language (Chinese, Greek and Latin in the future), Music and Art once a week.
I know finding a curriculum can be overwhelming. You might try to fulfill LA, math, social science and science first before adding other subjects.
I go to library resale shopping all the times over the summer and I have picked up many nice homeschool books (not curriculum, just history and science readers etc.) for 50 cents to a dollar. My library calls it Friends of the Library Bookstore. I don't know whether your library has it or not. Not all library resale bookstores have good books. I have been to a few of them before I found one that is good.
