Use baked baking soda. Bake it at 420 for about, well I do it for 30-45, and you get a very base powder. Then mix the baked stuff with the baking soda at a 1 to 6 ratio respectively. Makes a great ~8.2 PH buffer.
If you don’t want to have to bake your baking soda to use as buffer, you can mix 6 parts of Baking Soda with 1 part of Washing Soda (in laundry section of supermarket/Walmart). Baking soda is NaHCO3 and washing soda is Na2CO3. When dissolved, you have both HCO3^-1 and CO3^-2 which is essentially the same as the commercial buffers with the exception that the commercial buffers have some borate in them I believe.