• Amazon
• Barnes & Noble (and B. Dalton and Waldenbooks, but their online pages reroute to Barnes & Noble)
• Diesel
• The Regulator Bookshop (in Durham)

Also check out IndieBound to find out where you can buy the book locally!

• Amazon (Kindle)
• Smashwords (various formats)
• Barnes & Noble (Nook)
• Apple (iPad, etc.)
• Kobo
• Diesel (which is now partnered with Kobo, so go there instead)
• Baker & Taylor (which you can search if you have library status)

Also available with a subscription to Scribd and possibly at your local library.