Have you checked ebay? I notice there are usually something like 4 or 5 speedo/gas/clock, or speedo/gas/lights clusters with an odd speedo/gas/tach thrown in as well.
Do a search for 1971 Mustang, then select the 'Vintage Parts' in the filters on the left - it should drop down to less than 700 or so entries (weeding out the new seat covers and JDM cheapo steering wheels).
Hope that helps.