People get all bent out of shape about a little air in the bottle, it is about 20% Oxygen, and what ever it is going to do is nothing compared with the flow of fresh supply air through most caps, -- actually every one I have used aside from fitted glass stoppers. If it keeps you up nights buy some of the heavier relatively inert gases which can be introduced in to the bottles. Do the math, not that many oxygen molecules in a reasonable amount of air over the liquid.
Off the top of my head, at STP, V (in liters)/ 22.4 L x 6.023 x 10 e 23 x .20 % - - not to obsess or anything.
Yes, I used to sell bottle caps, from poly seal to corks, and the only ones that did not breathe air to the solution were glass stoppers fitted to the bottles, or, most others with Saran Wrap under the cap, -- not some other brand, Saran Wrap is denser and will extend the life of your solutions. You can test some samples and bottles-- I have, and so has Ctein in years past as I recall, Saran Wrap works.
Well, a good wine corker, fresh corks, and wine bottles should work as well, paper labels will stick with milk and you can celebrate vintages of various mixes?
If you are mixing up a gallon, and not processing much, you can easily find four one quart glass bottles, like the Boston bottles used in labs, and use the Saran Wrap .
There are plenty of catalogs for bottles. Many plastic bottles breathe through the plastic, some do not-- and the main problem returns to the caps.
D76 is rather easy to mix from scratch, if you have stocked up on the chemicals and have a good scale-- if you want small batches at very low cost.
Flynn Scientific and Carolina Biological used to sell a lot of glass ware to schools, and I have seen catalogs from back in my bottle selling days.
I mostly used it diluted 1:1 and tossed.
Regards, John
ps-- looking for a good scale, Ohaus triple beams are often sold by police stations at auctions after they finish with them as evidence from the kids selling pot who probably stole them from the local schools, or they should be around $100, electronic scales should be even cheaper. I used to cable them to the desks so I could have them for the next class.