Schirra's signature varies widely, and whether he uses a ballpoint, or felt tip makes quite a difference. In his many years as a space commentator with CBS, and even later, his signing habits varied greatly. For a while he'd sign everything, then for a long while he'd return everything unsigned, then he'd start up again. Currently, he has a contract with a private company which must be quite strict as I don't know of anyone who has had success in recent years writing direct. Nevertheless, I include his address in case you want to try.
Most astronauts are pretty good about signing their books.
Walt Cunningham, whose signature hasn't changed much through the years, also has commercial ties, but does sign from the address I've listed. For years he freely sent out signed NASA lithos for a SASE, so I'd imagine if you asked him nicely to add something like Apollo 7, or the dates of the flight, or to personalize that he would comply. He was always nice to space collectors. The late Donn Eisle was also good to collectors, but he passed away many years ago and the supply of his autographs is not great. Gemini IV veteran James A. McDivitt was again in command and was the first man to pilot the LEM in space.