Peroxide is best, you can buy it in bulk on that well known auction site, bleaching powder you can buy from the likes of Bushwear OK if you are only doing a couple of heads but works out expensive if you have more than that to do, same with the cream from the hairdressers , as I said you can buy it on the bay in bulk and in various strengths , while the stronger ones are not necessary they do get the job done quicker and you use less in the long run.
Be careful if using the really strong ones they can be quite nasty, there is a limit to the strength the general public can buy now,I think it's 30% anything stronger is for industry only and there are regulations on storage etc.