If you need to find items to include in your collection, consider searching through these sources:
Launched in 1996, AbeBooks is an online marketplace where you can buy new, used, rare and out-of-print books, as well as cheap textbooks.
Though marketed as having everything "From A to Z," this online retailer often yields good information about texts.
Google Gooks allows you to preview books and get a glimpse of the contents, which in turn equips you to make better decisions about which items to purchase for your collection.
And don't forget about Project Gutenberg, the online home of free texts! Project Gutenberg offers over 42,000 free ebooks. Choose among free epub books, free Kindle books, download them, or read them online.