I do understand that you're supposed to take food grade. However, the 3% solution from the drugstore works well also. People imagine that somehow it will hurt you. How is that possible? The cheap stuff is made to put on an open wound, which means it will enter your blood stream. It is also recommended as a mouth wash. You really think that no H2O2 will enter your bloodstream while swishing it around in your mouth? Of course it does. So while food grade might be ideal, the cheap stuff is safe or you wouldn't be able to survive pouring it on open wounds.
The label on the 3% drug store version says you can use it as an oral debridement but not to drink it. Imo, that label is simply there to protect the manufacturer from being sued by people who think you should be drinking H2O2 like soda or water in massive amounts. It is inevitable that you will ingest a few drops each time you rinse your mouth with H2O2.
Also, don't fret if you can't find 35% solution. You can use a lower concentration but you just have to increase the dosage. E.g. if you get the 12% concentration, then you just triple the number of drops in the same amount of water (3x12%=36%). Same for the 3%. Just multiply by 12 x.