The killing of microbes in the soil with H2O2 is simple really there are aerobic and anaerobic microbes in the soil the anaerobic (sorry about spelling today had a slight stroke yesterday) are poisoned by the air they live in the soil when there is a lack of O2 and are usually pathogens to the plant and other beneficials in the soil. The aerobic microbes are the critters which are beneficial to the plant and soil. They are invigorated by the extra oxygen thereby breaking down the soil and nutes therein to feed the plant.
The expense well, in the states the cheapest place to buy it is probably beauty supplier, and they no longer sell 35% in the US because it can be used in explosives. Here are a couple of links you can look at, there are many more studies and forums which talk about the benefits and drawbacks, I am not trying to sell a golden bullet or anything just something that over the years I have added to my regular routine in the garden and grow room.
http://educate-yourself.org/cancer/benefitsofhydrogenperozide17jul03.shtml
http://www.quickgrow.com/gardening_articles/hydrogen_peroxide_horticulture.html
http://www.gardenzine.co.uk/peroxide.html
Ok, not to get into some biological pissing contest, I must point out that you are incorrect on a number of your assumptions. The first being that h2o2 only affects anereobic organisms. H202 is toxic to a wide variety of organisms, both anerobic and aerobic. You need to know what Hydrogen Peroxide does to SOME bacteria. It is, for some bacteria, considered as a metabolic poison, which kills them unless they have the enzyme catalase.
Hydrogen peroxide is NOT SPECIFIC, meaning there's no exact way to say that this or that organism will be killed by hydrogen peroxide or not; some organisms, aerobic or anaerobic, can withstand hydrogen peroxide and some cannot. The only way to know is to do the Catalase test (hope you know what this is). Catalase is the enzyme that some bacteria use to catalytically cleave hydrogen peroxide into water and oxygen...so, no hydrogen peroxide, no death to the bacteria. Again, hydrogen peroxide can affect any bacteria, whether they are aerobic, anaerobic, or facultatively anaerobic--there's no way to tell how unless you do the catalase test. The only ones not affected, in general, are the ones with the enzyme catalase.
With respect to the anaerobic bacteria not dying, well, there are a number of reasons. Some "anaerobic bacteria" are just aerotolerant, meaning they can live with air, but only in a certain partial pressure of oxygen; some anaerobic bacteria are not really anerobes at all because they are actually facultative; and anaerobic bacteria do not necessarily die that easily once exposed in air--however, if you grow/culture them in the presence of air, they surely will not grow.
So if you are spending money to inoculate your soil/ hydro garden with beneficial microbes and fungus..and then dumping h2o2 on them, you are in effect wasting your money. While 3% hydrogen peroxide is pretty weak in terms of an sterilizing agent (and as an oxygenator for that matter) it does technically increase oxygen levels for a short period of time.
The other big error is the assumption that 35% peroxide is no longer available. Ummm, have you been in a grow store lately? I bought a gallon bottle of it last week. I use h202 to treat my grow medium between cycles. I use high concentrations of it to sterilize my grow medium. It is a very common hydroponic practice to add 35% h2o2 to your reservoir if you can not maintain proper temps below 72 degrees. the h2o2 will add oxygen levels and keep harmful bacteria that grow above 70 degrees at bay. These people are effectively growing in a sterile environment. This takes some of the advantages of beneficials off the table but also eliminates the risk of root pathogens hurting your yields.
Again, your roots need oxygen for sure. If your growing in soil then I would suggest just running a courser medium, something that will allow for better gas exchange. I really don't see how you could get oxygen pumped into a soil type medium to dissolve in a efficient and effective manner. If you are not overwatering then your oxygen levels should be fine.