I found mine in a parts car at a used Mustang place. I see them often on Craigslist and ebay. Classic Auto Air has them available as well. Call and ask them if they have a used case available that you can buy along with their new High efficency Evaporator. The rebuild is pretty easy although you may have to make some Fiberglass repairs on the case which re often cracked. You'll need:
67 or 68 Case (They are different but interchangeable) BTW they are available New
Blower motor asssembly
Defroster assembly (Available new)
Side vents (Available new)
Control cables (Available new)
Control Hose (Available new)
Vent Hoses (Available New)
Hose retainers (I made my own)
Water Cutoff valve (Repros available)
There are lots on Ebay although the prices may not be cheap.
67-68 Ford Mustang AC/heater box & plenum with motor, C7ZZ-18A409, EXCELLENT | eBay
CORRECTION:Oops, overshot my 4 min limit for editing!
The new one looks nothing lime the original on ebay.
Sorry looked at the wrong of the two links. The new one at $325 looks like dead ringer to my original.
The used one with the attached fan looks nothing like mine.