I cannot speak for the original poster, but I see them all the time on GunBroker. Search for Benelli tube, and you will find what you are looking for.
On a seperate note, if you have an M4 that originally came with a fixed pistol grip stock, the OEM recoil tube is milled in three positions for the collapsable stock. I only know this because I almost bought a new recoil tube, only to be informed by Benelli Tech Support that M4's without a factory skeletonized stock have milled recoil tubes.