I got mine at lowes (kobalt brand), and I used some j-hooks I had here to mount it. it has been in 2 trucks now.
I also recommend the toolbox drawer lowes sells. it mounts tothe bottom of your existing toolbox, is lockable, and adds a ton of space for storage.
I like the kobalt ones cuz they seem to be made better. the ctd had a deezee box on it when I bought it, and it was a piece of crap... . not trashed bad, but the build quality sucked. Where mine is fully welded, it was only spot welded.
You are only supposed to mount the toolbox to the truck with one bolt on each side. this may be causing some problems if the toolbox adds too much stiffness to the bed.