I have a Pinarello purchased in 1987 that has Campagnolo brakes. Each of the brake pad holders have a v-shaped piece of metal covered by black plastic. The holders are stamped Brev. Campagnolo. The block has four square shaped pads, and they are better than finger nails on a blackboard in alerting people that I am around when I hit the brakes. I'd like to get replacement pads to keep the bike as original as possible, so I'm hoping that if Campagnolo doesn't make new stock that there may be a third party that makes pads that fit these holders.

Thanks for any suggestions.

You can easily find and buy Campagnolo replacement pads for your brakset from eBay sellers.
Those pads aren't anything rare or unique in the Campagnolo product line and was used on many brakeset models for many years. Just shop by sight and you can easily find a match for those pads.

Alternately, you could use black Kool Stop pads made for those holders. They look nearly identical to the Campy ones, which are available too. That should get rid of the squeal.
Carefully clean the braking surfaces on the rims too.

What Chombi and Rootboy said, Campagnolo's replacements, and Kool Stop's pads, both will do the job quite well. The Kool Stop are available in the black or a softer, wet condition, salmon colored pad. If the squealing continues you might have to chamfer the pads and remove the glaze that they have when new. Some fine grit sand paper, a Dremel tool works well with the drum sanding bits, and just rough the flat up somewhat. You may need to angle the faces so that the pad doesn't hit the rim parallel, but the trailing edge comes in contact first. The Campagnolo replacements look very much the same, but the blocks now have an "X" cut, rather than the separate squares. I just watch eBay for the originals, or use Kool Stop's Salmons (both are in use right now.)

Some folks prefer to bend the brake arms, I wouldn't advise using that method, those are Super Record calipers and are really getting expensive to dig up. NOS ones can run you upwards of $350.00 or more, without the pad carriers and levers.

I'd buy the KoolStop blocks. I also vote for cleaning the rims real good. Use some rubbing alcohol to get all the dirt and oils off them.

Kool Stop International - High Performance Bicycle Brake Pads Since 1977