Try Ebay or Craigslist, however for 5K you need to be aware that you will likely need to invest several K more in order to get it into a daily driving status. .
The 64 1/2 is still a collectors car, particularly the convertible and they are still big bucks, the 67 not nearly so much. Craigslist, does it have a "wanted" section? I just found exactly the older Nissan I want ed there.
Check Craig's List and EBay - as long as your expectations aren't unrealistic (matching numbers, K code, etc) and you're willing to do some restoring, you shouldn't have any problem finding one for under $5K.
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, horsepower, modifications, troubleshooting, racing, and more!