I used to be able to buy 35% which, due to the shipping weight, was the most economical means of buying it. But since Federal law apparently now requires merchants of 35% concentration hydrogen peroxide to be licensed, and hydrogen peroxide over 20% solution is part of the Items never allowed by the USPS, then it is hard to find. http://www.dfwx.com/h2o2.htm does offer it, though I am, suspicious of the legality of selling it for personal use, even though I do not like the restriction if it exists.
The link I sent in comment #30 is where I just go 2 gallons of 35%.
I buy all the time. Good shipper (blubonic.com)
They ship USPS ..mine came in 2 separate boxes.
Each gallon in plastic bottle then double wrapped with strong sealed plastic. Be careful as can burn your skin at that strength. Rinse quickly if you get some on your hands.
Good luck