3% H2O2 is the fancy scientific smart stoner way of saying Hydrogen Peroxide. Go to your local drug store or anyplace that sells medical stuff and buy a litre of Hydrogen Peroxide (3% is what is most common), it is cheap and only costs like $2 for a litre.

I am not very fimiliar with the Ebb and Flow system (I know how to build em and work em, I just don't have one), but I think you should just get a cheap rubbermaid 18 gallon container, fill it with 2/3 water and H2O2 (2/3 cup or so), then stick your pump in and run it. Let it fill up the system and let it run for a bit. The flow of cleaning water should wash out the system, sterilize it to a point, and get it smelling clean again.

Then after that, use a fresh batch of Nutes & Water and run it for a short time just so your plant's roots will be left with nutes instead of oxygenated water.