I found the whiteboard stuff at Wal-Mart awhile ago, in the stationary/home office department. It was quite inexpensive. I think it may only be stocked seasonally, perhaps at back to school/off to college time.
The safe is screwed down to some wood blocks that are screwed down to the floor. The wood blocks were needed to raise it high enough to clear the structure/framing for the bed. I used what are sometimes called "furniture bolts" similar to
these ones to bolt it down; the big head provides a sturdy mounting, yet it's thin enough to avoid interfering with the interior of the safe too much and doesn't have sharp edges to catch on things.