Online Book Sources

Google Books
A good place to find a selection of free and low cost eBooks.
Internet Archive
A free internet library of eBooks, audio recordings, and other media types. While not all inclusive, there is a decent selection of 18th century related materials including primary documents.
MET Publications
MET Publications is a database of art based books provided by the Metropolitan Museum of Art. Many of these books are available as free downloads.
University of Wisconsin Digital Collection. Digital Library for the Decorative Arts and Material Culture.
Includes various books about decorative and material culture from the 17th century through the 19th century.

Book Purchase Sources

Amazon
Often carries reprints of 18th century books. Also is a good source for out of print books that are being resold by owners. A search in Books for “Gale ECCO” will produced print versions of many books found in the elusive Eighteenth Century Collections Online
eBay
A good place to find a selection of free and low cost eBooks.
Kings Arms Press
Specializes in fine reprints of 18th century books and pamphlets. They carry mostly military books but they do carry some civilian books and pamphlets as well.
Revolutionary Imprints
This is a good source of Revolutionary War primary sources of various kinds. These sources are on purchasable CDs.