So, you bought the loader from a dealer, or someone other than the original owner? Any way you can locate the original owner to see if they still have the brackets?.
I looked through my photo archive to see if I had any closeup images, but I couldn't find one. These brackets are pretty thick, best I can remember they are 1/4-5/16 " thick, are right and left side paired, have two bolts each into the frame rails and one bolt each into the cross bar. They were also a PITA to install or remove, so maybe that's why you didn't get them with the rest of the stuff. If you could find pictures of them, and can make a scale drawing with closely measured dimensions of hole centers, a good machine shop could most likely make a set for you. It would be a shame to not be able to fit the loader, it was one of the handiest things I had for my X748. I would have a copy of the manual and assembly instructions, but they went with the loader when I sold it and the tractor.
Whatever you do, don't even consider just having the bar welded to the frame. There are a couple of things you will have to do at some point, such as replace your alternator belt, that will require removing the crossbar for access under the tractor. Also, once you mount the bar, the triangular tabs face point up from the back side of the bar, so they fit properly into the loader masts.