If you're unable to get them you can just use nuts and bolts to hold the grates and the bowl. The bowl or diffuser can sit on the third grate down.
Or perhaps you can fabricate the straps from aluminum bar stock. They seem fairly easy to configure. I personally would just stick to nuts and bolts like most every UDS build. They'll accomplish the exact same thing.
....UDS....Mini-WSM....Midi-WSM....Mod-ECB....22.5-OTG....34"wood fired pizza oven....