Curriculum is a very interesting thing. There are companies that market and publish materials solely for public schools, some that do the same for private schools, some that do the same for homeschoolers. There are also publishers that try to market and publish for more than one of the groups. Then, there are also companies who get noticed by homeschoolers, but do not avail themselves of that market.
Harcourt/Houghton Mifflin publishes the Saxon phonics and Saxon math curriculums. These are used by public, private, and home schools. Teacher's guides can be purchased for these curriculums by homeschoolers. Their public school materials on the other hand (which I use and buy used via Amazon) are generally more expensive than other homeschool alternatives. As a certified teacher, you can order curriculum from Harcourt and get the school price for the materials which is discounted from the regular price. The same goes for Macmillan/McGraw-Hill. If you are not a classroom, certified teacher, you cannot order teacher guides from the company. But, if you are a certified teacher, you can also get the school price.
Evan-Moor and EPS books are two other companies used both by classrooms and homeschoolers. The prices are the same for both. Evan-Moor does have a mailing list that sends out free shipping offers, I believe if you sign up for it on their website. Evan-Moor's books are great because they are reproducible and can be used for more than one child. They also have student books for a lower price on their website (these aren't sold on Amazon). I use their Daily Language Review and Daily Math Review to supplement my math curriculum because I use a mastery curriculum, rather than a spiral curriculum. If you haven't heard of EPS books, they are the company that publishes Explode the Code and Wordly Wise (both wonderful series). EPS is a great resource because their materials work for both learners both above and below grade level. Each of their series do not identify their books by grade levels, but rather by letters or numbers (which are not associated with grade levels).
Handwriting Without Tears is a company that I had assumed was geared towards homeschoolers. It is actually not. It is one of those companies that has written materials for the classroom setting that was noticed by homeschoolers. You can order their materials for your children directly through their website. This year, they came out with a preschool curriculum that includes three teacher guides for $20 each. Many of the other preschool materials are not affordable for the average homeschooler, like me. I recently inquired of the company to ask if the weekly plans from the teacher's guides would work for homeschoolers. My concern was that though the guides are affordable, would they need the other materials they sell in order to teach the lessons? Adding in all the other supplemental materials would make the preschool materials quite an expensive proposition. Based on the email I received, I believe they wouldn't work for homeschoolers. They are marketing and gearing their materials to classroom settings.
Up to this point, I have chosen to use many public school materials that I have found online. I have a list of what I'm searching for that I keep an eye out for all year long. I have a feeling that as we head into middle school, I am going to need to choose more texts for homeschooling simply because I will need the teacher's guides. Teacher's guides for homeschooling materials tend to be much (!) more affordable than the ones written for public school materials.
But, for now, Evan-Moor, EPS and Harcourt have a lot of books sitting on our shelves.