I had the same question on here back in January that appears to have been deleted. The only source I could come up with was to have them custom made by Briley at around $65 a piece and a 8-10 week wait.
Smith and Wesson was no help, I was told to "google it" by their customer service.
The gun was made at the Akus factory in Turkey and
http://www.commando-arms.com sells their current models. I got a reply from Akus through their Facebook page and was told that they would have a US distributor contact me but it never happened.
Also check out
Webley And Scott | Shotguns Looks like the same gun. I called them but they told me they don't sell the chockes separately.
I'd be happy with one complete set. I bought my Silver Elite used and it only came with two chockes, M and IC.
If you have any luck I'd love to hear what you find.
Dave