As you've probably realized, these are tough to find as a result of:
-being made only for the '66 Beaumont
-being made of crappy plastic!

This is why some have opted for '66 Chevelle bumpers which don't have reverse light holes. (Others I believe have gone to '67 Chevelee bumpers with '67 Chevelle reverse lights?)
I was able to get some old LeMans lights from one of the members here which came pretty close to fitting the bill, however they did not quite fit flush enough. Too bad considering they are metal, and reproduced. (Oddly enough, the Beaumont reverse light covers will fit into the Lemans reverse lights, but only backwards as they are tapered.)

Good luck with your search...

maybe here

1966 beaumont 4 dr v/8 auto parting out

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View larger image   Date Listed 06-Jan-10 Price Please contact Address Saskatchewan S0L 0P0, Canada View map For Sale By Dealer Make Pontiac Model Other Year 1966 Kilometers 11111 has none running 283 auto , good grills ,dash some body parts ph 1 306 493 2637 can not return emails

427carl wrote:

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maybe here

1966 beaumont 4 dr v/8 auto parting out

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View larger image   Date Listed 06-Jan-10 Price Please contact Address Saskatchewan S0L 0P0, Canada View map For Sale By Dealer Make Pontiac Model Other Year 1966 Kilometers 11111 has none running 283 auto , good grills ,dash some body parts ph 1 306 493 2637 can not return emails

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You can try him, but that's where I got my two.  (I didn't ask him if he had four, shame on me.) 
I do have a few other pieces coming from him. (Hub caps, and hopefully the grille and parking lights.  Don't ask for those though as you'll drive his price to me up!)

His name is Rick.  A nice fella. 

I have a couple that I picked up from Tin Man on the island. They weren't cheap, especially considering that they have holes where the bulb sockets are to go. It's insane what you have to pay for these, but there really is very little supply because they crumble apart. My plan was to epoxy the sockets in.
In the long run, I'm really hoping that we can find an alternative metal repro from something be it Ford or whatever. (When I go to car shows, half the time I spend checking out various reverse lights!)
I would suggest you call Rick (even tonight as that is his cell phone.) If you exhaust all options, let me know...