Recognizing the cost and difficulty in finding a correct original antennae, any suggestions for a more reasonably priced/available alternative?
It is a tough part. It has been that way for decades. Be aware that lower priced(4-600.00) exact looking antennas come up for sale every so often however the parts that they are made with render the antenna inoperative . It looks concours correct but will not allow radio waves to reach your radio. It is for looks only. Here is one on Ebay
https://www.ebay.com/itm/324694498055 . Owners and restoration shops in the past replaced a broken or missing factory antenna with a Ford accessory full size car electric antenna . It looks similar but not exact to the top side of the collapsible factory antenna. Trunk side of course looks different. The switch is installed in a convenient ready made factory hole above the radio trim panel. Another replacement that is sold as for Shelby(not) it is a Ford collapsible antenna it has a C5AZ J engineering number.
https://www.ebay.com/itm/402722926722 . It is for Galaxie. It is different in every aspect mast,tip lower half and coaxial cable but still looks similar.