I found the balloons I hid from myself. Purchased on eBay about a year ago.
Current listing seems to be item: 381306135647
Mine are smaller than the ones Rich described. I picked the smallest one I could find which was: silver, 5" heart. They didn't offer 5" round and the heart shape would probably have less volume, which seems like what we want.
Today, I measured some parameters. To measure the volume of the balloon, I used multiple injections of air from a 60 cc syringe I had on hand. I found a plastic bottle top for squirting its contents that had a tapered tip. The tapered end of this cap fit snugly into the filling tube of the balloon. I secured the balloon with a little piece of tape for this test. I had some 1/4" OD plastic tubing that fit nicely on the end of the syringe. I was able to jam this inside the taper of the cap to be secure enough for this test.
In this testing for volume, I discovered that balloon technology is more advanced than when I was a kid. These Mylar balloons have an internal one-way valve to hold the gas inside. This was good for my syringe filling process as I had to disconnect the syringe each time I refilled it with air, but the valve, for storing D2, needs to be defeated so we can get our stored gas back out.
Here's the configuration I used to fill and measure the volume of the balloon.

- Filling measurement of volume
What I found is a tightly filled balloon has about 400 cc of gas, but this is a bit of positive pressure inside, above atmospheric. If the balloon is filled to just about atmospheric pressure, a full volume seems to be about 300 cc. If I did the math right, this would require electrolysis of about .25 g of D2O or about .22 ml.
Here is a picture with the dimensions of an inflated balloon.

- Balloon dimensions
I measure the inside diameter of the inflation port of the balloon at 6 mm or .236".
I sacrificed one balloon by cutting it open so I could see what the one-way valve inside looks like. Here's a picture.

- Internal one-way valve
Its a flat tube about 3" long inside the balloon formed by two flat pieces of plastic film, sealed along their longer edges. Any pressure inside pushes the two flat sides together, keeping it sealed for any gas wanting to pass outward. This was handy for my measurement inflation process, but not good for getting stored D2 back out.
With the balloon fully inflated from my measurement, I was trying to figure out how to get the air back out. I have some 1/8" plastic tubing that is flexible but a bit stiff. I forced that through the internal valve to get a drain path. If that was done with the balloon empty it would have been easy, but with the pressure of a full balloon forcing the valve sides closed, it required some back-and-forth twisting while applying gentle insertion pressure to work the tube inside. For this balloon about 4" length of the 1/8" tubing into the filling port seems to be about right to keep it open.
My first thought to permanently defeat the valve was to insert a spring hook and pull the valve out to cut it. This might work but would probably be difficult and it would be easy to damage the balloon in the process. I think keeping a piece of the 1/8" tube inside the valve is probably best.
So my plan for a permanent storage is to turn a metal tube .236" in diameter to fit into the inflation port of the balloon. The inside of this will be drilled to accept the 1/8" tube about 4" long, to bypass the valve, and it will be glued to make the attachment. The outside end of the .236" tube will be soldered to some kind of plumbing fitting, possibly a 1/8" compression fitting. Once assembled, this adapter will be inserted into the balloon port and glued, probably with epoxy.