William or Karen : Help..I want to go from Blonde to brown...

I currently use L'Oreal Excellence Blond Supreme Ultra light Goldblond (No. 02). My hair has become very porous and I know from reading this website that I need to use a filler. I want to stay blond, but would like a bit darker blond. What I don't know is where I can purchase hair filler. Is it available in stores, or do you have to have a license to buy it?I know your first suggestion will be for me to go to a professional to have this done which I did two months ago. The first week the new hair color was very dark, but it faded after a few weeks back to the original pale blond. Needless to say, I was very disapointed. I felt I could have had the same results if I had done it myself at home. So, I would like to try it myself, using a filler first, only I don't know anything about fillers.I would really appreciate any help you could offer and I will be eternally grateful!Thanks in advance,Kelly