As the Apollo astronauts get older, more stuff comes up for auction. Some of it is fairly affordable (like signed panoramic photographs, check out these Quicktime VR views) or Cernan's famous picture of the "full earth".
If you want to go retro, pick up a Chesley Bonestell print, like this famous one.
I don't get it, but some of them have taken up painting, like Alan Bean (he signed a book for me at the Tamsen Munger gallery) and Michael Collins, who seems to like painting fish. I was really impressed with what he had to say in the documentary In the Shadow of the Moon. That movie was not at all what I expected.
I also don't really get autograph collecting, but if you do, watch out for autopens.
I would rather get the Apollo 14 scoop used on the moon (minimum bid of $125k, expected to go for at least a quarter million dollars), the Apollo 17 water gun used on the moon to rehydrate food, a Saturn rocket model signed by Wernher von Braun, or gingerbread cookies that are well into their fourth decade.
Some of the Apollo astronauts got into ESP or religion, a topic for another day :)