Can I purchase just the history curriculum?

Yes and no.

You can absolutely buy just the History titles in any given package plus the History / Bible / Lit Instructor's Guide for your schedule and notes. But Sonlight also uses Read-Alouds (and, for levels D and above, the books your student reads) to further enhance your study of history. Which is why Literature -- Readers and Read-Alouds -- is bundled with Bible and History into one package at a great discount with a fantastic guarantee and more.

So, no, Sonlight does not have a complete History sub-package for you to buy, but you can opt to purchase just the History books (and corresponding IG) or the complete History / Bible / Lit package. The choice is yours.