Such items are available for purchase, without restriction as far as I know.
Did I mention that it is a bad idea? The only rational argument I can think of for such an item is that due to its shape and design, it is likely to be mistaken from a distance as either an LE insignia, or a security guard insignia. I see neither of those impressions to be legally helpful, though they may be effective as to the intended purpose.
And this is a topic that has been discussed at length. Multiple times.