If you have a specific celebrity that happens to have an official website with a contact e-mail, before you send out any LOR to them (sometimes they don't even have an address posted for the public, so this really helps!) send them an e-mail asking if you sent a SASE (and a photo if you want) would they be able to sign it for you? They usually get back to you very fast and tell you that they would be more than happy to along with the actual address you would need to send it to.

I just got e-mails back like this when I woke up this morning from Richard Epcar (Ansem from Kingdom Hearts as well as other animes and video games) and Vince Caso (Bladezz from The Guild).

It's also good to check official websites because sometimes it will say that they definitely do or don't do TTM autographs. I know Emily Giffin (author)'s website does as well as Susan Egan and Paige O'Hara (voice actresses).

Make sure it is their OFFICIAL websites/e-mails though. Usually it's very easy to tell. Just be aware.