In my website’s store, I carry hardcover books that have been out for at least a year (I don’t want to compete with bookstores on new releases). You can ask for a simple signature or a personalization. I sign orders about once a week when I’m not on tour, so it may take a few weeks to get done. Signed copies can be purchased at http://store.brandonsanderson.com/signed-books/ .
You can also check Upcoming Events (http://brandonsanderson.com/upcoming-events/) to see if I am doing book signings in your area.
I often sign extra stock when I’m in a bookstore for a signing, or just passing through (like in airports). You can call stores I’ve been in, or I’m planning to visit, to see if they’re able to either hold or ship a book for you. I often will do a special signing at Sam Weller’s in Salt Lake City near release day for mail order customers.