I couldn't tell you the technical name,but it is definitely
available from All Glass Aquarium.It comes in lengths so you may need to cut it to size.It is easily obtained.You need to go to an aquarium specialty store,not a fullservice petshop.Good luck!
John

Well I stopped on what I believe was the correct website www.allglassaquariums.com
but I didn't see a catelog rather it seemed to consist of links to other sites.

Allglass doesn't sell directly to retail customers. I've seen the plastic back sections at aquarium specialty shops, but never at a general pet store. The hinges and handles, to allow you to make your own tops; glass, acrylic or a combination of both, are available at: Glasscages.com. They don't seem to list the back panels, but have everything else to build a tank from scratch, so might have the back panels available as well.

Look at the list of items on the left hand side of their home page and click on "Tank Frames."

I've given up on the flexible back sections, as they crack, discolor and are overpriced. It's a lot cheaper to just cut a piece of 1/4" acrylic to fit and acrylic panes in various sizes and thicknesses are available at any home center.

However, using acrylic for back panels requires another hinge, to allow the acrylic to be attached to the middle glass panel, but unlike the plastic back panels, the hinges are cheap.

But that's a point of view from an aquarist who keeps dozens of breeder tanks. If you have but a single tank, you might be as well off just replacing the entire glass top, or go to a hood.

Also check out Foster & Smith.

Ah Ha! Foster & Smith it is. See their item # CD-931079.

A 6 ft. length of back panel @ $5.99.

Not as pricey as I thought it would be and a darned sight cheaper than it was at an aquarium specialty shop.