No, there is no source for those corners other than savaging from other amps. I ended up buying a doa stinger just for the corners. Those corners were made to SUNN's order. They cut a corner off of a Kleenex box and used that as the original.
Here is a message from member Rex B from some years ago...
Hey Steve
No problem, you are welcome to send me messages directly through here, and I am more likely to find them than if you were to post them on the forum.
The cabinets on your Enforcer gear were designed slightly after I left Sunn to work at Fender, but I am somewhat familiar with their heritage. The cabinets of this final pre-Fender era were made by a company in Troutdale Oregon called "Bennett". They were done using a technique called Miter-Fold. I looked for Bennett Cabinets in Troudale on Google, and did not get a hit, so they may not be there anymore. I THINK the edging is a stock vinyl extrusion from some plastic fabricator , but unfortunately, I don't know who.
The corners have a more interesting story. Larry Davis, the mechanical engineer at the time, told me he took a plastic stacking Kleenex box, and cut the corners off at a 45 degree angle, (probably on a bandsaw). They used these as a pattern to make an injection mold for the corners. This is why those corners have kind of a sharp edge on them, and undercut toward the cabinet: they really should have beveled them the other direction so they didn't catch on things when you try to slide the cabinet. People have also been cut on them.
Regardless, I'm pretty sure Sunn owned the tooling on these, and they are probably not available through a 3rd party. You MAY try to contact Fender to see if you can get any from them, it's possible they may have some old stock. They may also have some of the edge molding, but I am pretty doubtful on both counts. I do suggest that you put a post on the forum asking if anyone has the molding and corners. Some of the employees from that era of Sunn may be more familiar with the suppliers. I would also suggest sending a message to Don T on this forum, he was involved with purchasing and production during this time.
Hope this helps, and good luck with your Sunn gear !
Rex B