You can try some of the lists in the Resources tab at the top of this page. You can try suggested lists at sites like
Time4Learning. (Click on the grade levels in the right sidebar). You can also do an Internet search for "third grade spelling words" or "third grade spelling lists" and find many that way.
If you use the resources on this site, you might want to start with the Dolch words. Also look at the literature lists. It's fun to read a book and then have spelling words that go along with the book. Most of the literature word lists include a suggested grade level. Don't be afraid to use second or fourth grade lists, depending on your child's ability. :)