this sites store, amazon, kindle, any book store. depends on if you want a physical copy or not. you can also check if certain books are in PDF format. Bucklands Complete Book of Witchcraft is in PDF and that's a good one to check out.
of course you can always read articles, or google definitions and other information and build your Book of Shadows that way and simply study off it.
This site is filled with a variety of resources to get you started. Alternatively, there are many books listed on this site and mentioned in various forum posts that can direct you to what you are looking for.