The best place is to buy direct. That way your purchase doesn't go mainly to Jeff Bezos and you will get a signed book. The first trilogy (which is where this excerpt is from) is the "Enemies (Foreign and Domestic, Domestic Enemies, Foreign Enemies) series. Here is Matt's Gab page with details (BTW if sending Paypal make sure you include your address because it is not included in a Paypal autofill in the order.
Matt Bracken Gab Home Page with book ordering info