You could probably find the volumes in any decent book store since the series is really popular.

You can find the first 11 volumes on Amazon all in English (with 12 being released next week). Most of the volumes are around $6-7 at the moment.

If you don't mind waiting there will be a special edition of the manga released in English called Attack on Titan: Colossal Edition which will contain the first 5 volumes (1000 pages) and 15 color pages. It comes out May 27 and the current preorder price is $40 (from a regular price of $60) http://www.amazon.com/Attack-Titan-Colossal-Edition-1/dp/1612629717/. Worth noting that the size of the book will be 10.5 x 7 x 3 inches when the size of the regular volumes are 7.6 x 5.1 x 0.6 inches so it will be pretty big.