Up on Amazon (and probably bookdepository) is the sixth volume of the IDW hardcover series, for pre-order, to be out May 1st.
Regular retail price is US$50, but online sources like amazon have it for less.

The contents of Volume 6 include the Bumblebee miniseries, the first 6 issues of the ongoing, the Wreckers story, and the Prowl Spotlight.