The best place to get them is actually
Fictionwise with the coupons, where they're the same base price as Amazon, but the discount can take them down to less than $2 each if you wait for a 50%+ off weekend. And you get them DRM-free MultiFormat to work on multiple devices (though the Amazon ones should be DRM-free as well).
You can also get Elkins' Chris Norgren series and his standalone thrillers as well, for the same price. I picked up the entirety of Elkins' works late last year and earlier this year for roughly $30, I think.