Both the vacuum advance unit and the whole distributor should still be readily available new or rebuilt, and should be easy enough to find in junkyards as well. The 390 was one of a family of engines called 'FE' (Ford/Edsel) that were very common in their day, and at least one of them, the 360, soldiered on well into the 1970s under the hoods of pickup trucks.
If in a junkyard, look for a 352, truck 360, 390, or 428 from the 60s through early 70s. I don't know without looking it up whether the smallblocks (260, 289, 302), big blocks (429, 460) or 351C or 400M used the same distributor and/or advance unit.
I am able to get this stuff at the better 'regular' auto parts stores. If you strike out in your area, try the Thunderbird specialty retailers listed at
http://www.tbird.org
admin
PavelP