M7 bayonet. You could try the blades section of the EE.
An original name brand will demand a high dollar but some companies to make reasonable copies.
Not sure who sells them off the top of my head. I've spent a lot of times browsing the AR sponsors and never saw one.
One of our sponsors, on the other hand, did have a dummy plastic one if you just want one cheap for looks, I forget which tho'.