Hi,
Thanks for coming to the VFG for your search. Finding an original is a very tall order, they are quite scarce as you already know. And Mia wore a reproduction so ......
I do have a very workable suggestion if you have a costumer or wardrobe person who is handy with a needle. Find yourself a 1960s dress that has been trimmed across the bust, or sleeves, collar, or hem with rows of Aurora Boralis (AB) crystals. Mia's headpiece in the film used this very same technique of rows of AB crystals dangling. The end result will be quite realistic as the weight of the leaded crystals and glimmer will replicate Mia very well. And it is really fast and easy and the hard work has been done for you already by the original seamstress. Simply cut the rows of trimming and sew to your own fabric bandeau or cloche. Voila!
I have used this for several clients in the past, and everyone was very pleased with the end result. You can scour Ebay for 1960s evening or cocktail dresses that have damage and can be bought for a good price. I know there are some new (made in China) glass or plastic beaded "Flapper" headpieces on Ebay that may look similar and may suit your needs, but they look pretty fake in the hand and do not hang with the same dangle and weight as real crystals.
This is just a suggestion. Good luck!