Guardian is the manufacturer of the glass itself, but not the door assembly. See http://www.guardian.com/en/na/gd_002563.html
I think the key to finding the proper sliding screen is going to be finding out the manufacturer of the assembly. Have you looked all around the door frame for a nameplate?
I had basically the same situation in my 70's-built house, the closet doors went all the way to the roof, and were horrible looking bi-folds.
![before](https://i.stack.imgur.com/ECnNSl.jpg)
Basically what I did is framed in a header to bring the opening to 81" (remember to account for 1/2" drywall while framing):
![new framing](https://i.stack.imgur.com/yYbwVl.jpg)
Then some drywall and paint (note I didn't yet repaint the ceiling in this picture):
![drywall complete](https://i.stack.imgur.com/JhKoll.jpg)
I had 4 closets like this. One by the front door got sliding mirror doors, the rest got frosted-glass sliding doors. The right picture below is actually the "after" shot of the first one in this answer.
![finished with frosted glass doors](https://i.stack.imgur.com/Hle7km.jpg)
The width is going to be challenging for stock stuff. If it was a bit wider, you could probably split the door into two by putting a small column in the middle, but that would leave you with a couple 3.5' doors, which are pretty small. If you went to bi-folds, you might be able to do a 4' and a 2' door, though that might look a bit strange.
A quick search turns up some custom closet door manufacturers, which might be an option to get multi-panel sliding doors, like this:
![alt text](https://i.stack.imgur.com/ntVsV.jpg)
If you do this you can probably also get them 8' high, but honestly, even without the doors on, just making the opening 81" really updated the look of the house.
Yet another option is that you could shrink the width down to 72" (basically just extend the wall/framing on one side by 9"), and along with making the height 81", you're into a standard size where you can get both sliding or bi-fold doors off the shelf from any box store.
Good luck, and let us know how it turns out
Best Answer
In my area it is common in rental units to install panel doors in lieu of mirrored doors. We buy the mirrored door track, frame, roller, and hardware kits but with no mirrors. They call it a "knock-down" kit. We then cut wood or composite panels to fit where the glass would go. The manufacturer even gave us a table with net mirror glass dimensions, to use when sizing the wood panels. Check with a local high-volume window and door shop. You might even be able to get the kits from a "big-box" hardware store but I prefer to work with the guys at the sash and door shop for the personal service and expertise.
You might even be able to use your old frame and hardware, measure carefully first, then take apart the frames (they are screwed together) and cut wood panels to fit.