Some examples of resources for good sixth grade science textbooks are Houghton Mifflin Harcourt and Pearson Prentice Hall, both of whom publish textbooks for public or home school use. Other useful sources are companies that offer home school curriculum bundles and supplies, such as Oak Meadow or Timberdoodle.
Houghton Mifflin Harcourt's textbooks are available to private customers directly from their website. Pearson Prentice Hall traditionally circulates their products through major distributors, and their titles are available from some book stores as well as online retailers such as Amazon.com. Oak Meadow and Timberdoodle offer a variety of individual science textbooks from their online and print catalogs, as well as many extras such as workbooks, models and supplies for experiments.