Are you talking about what you see from the front - the piece that surrounds the instruments, the switch, etc?
If it's that, they are becoming difficult to find. I'm not sure if you'd be able to find a new one and it would be expensive if you did.
I see them on ebay, prices range from okay to ridiculous.
One problem with these is that they are soft plastic/rubber covering foam and used ones are often cracked, either from weathering or cracked in the thinner, flexible spots.
I don't mean to sound discouraging, just to let you know what you're up against.
They'll pop up on ebay, sometimes for a decent price but you just need to check frequently. A lot of people that chop up bikes or change to modern instruments will get rid of these.