The only "for sure" Garth address I know of is the one for GBM, but I doubt it's a good one to get an autograph from as I understand the mail piles up there.
There are some professional autograph companies out there where you can get a certified autograph which is less risky than buying something over Ebay.
I was blessed to have a GarthAngel in Sandytoo and she got Garth to autograph my favorite picture of him for me when she went to visit. Perhaps you can hook up with someone that is going to a Garthevent or meeting him some other way? If I'm ever blessed enough to get a chance to get him to autograph some stuff I'd get one for you.