I am not sure where to find the exact replica, however it appears to be a standard Perforated Metal sheet.
I have a Moon Patrol cab and I believe it's 100% identical to Joust except for art and CP. I measured my grille perforation sizes and sheet thickness and I found a close match on from McMaster Carr.
www.mcmaster.com9255T251
Steel Perforated Sheet 36" X 40", .0625" Hole Dia, 41% Open Area, 20 Ga In stock at $32.77 Each
It's more metal than you need, but it might come in handy for restoring other Williams cabs.