Sondheim signed only 8 posters, it's just his signature, no one else's. I am not sure if they sold any of those yet, but as they are so few if you want one I'd suggest going to the theatre tomorrow night at the end of the show to purchase one.
The signed invites (and everything else) are sold at the bar on the second floor of the Marquis, through the glass doors you have to pass through to enter the theatre. They are sold only at the end of the show, so get there right before the show lets out (9:30 on Tuesdays, 10:30 on other nights) to get what you want before the crowd.
"It's Phantom meets Hamlet... Phamlet!"