Hi Paul,
Are you wanting the mesh to keep the amp original, aesthetics? Or, do you need it to actually protect the valves from people, or people from the valves?
Unless you have small kids about the amp will operate fine and is relatively safe without the mesh protector. My MkII doesn't have one standard, and I've taken the one off my MkV (in storage). I guess Boogie have to put them on to reduce their liability and to keep us safe.
I think you will have trouble if chasing down an original replacement. Unless someone on this site offers you theirs you're probably easier to contact Boogie and ask if they can cut and bend a new one for you.