For the Usborne books,
http://www.usborne.com/quicklinks/eng/catalogue/catalogue.aspx?area=EN(These were cheaper ordering through a friend who sells these with facebook parties)
Usborne connects the information in the encyclopedias to websites you can visit. The links are noted in the book and each book has a webpage with the links listed.
Some other helpful sites for discounts on curriculum
http://www.homeschoolbuyersco-op.org/homeschool-math-curriculum/http://www.educents.com/For next year with the two kids, they are doing World History, Writing and Science together. Some curriculum save you money by having the same reading but different exercises and expectations for each level. I am going to use SWI B for language, I bought that for the older one but the website provides downloads for the lower level readings for younger kids without cost. For science, I bought everything for the older one and use that as the guide and use workbooks and encyclopedias to go along with the topic that the older one is studying so that my husband is teaching them both at once but their activities and tests are level appropriate. A number of history curricula have exercises for different ages in the same book. For math, one is using dreambox.com and the other is going to use the AoPS textbook on her own with Alcumus. I am not paying for the class.
Our area has a homeschool consignment store and a couple of facebook pages where people sell used curriculum.
