Oh, how you can be reminded of your impending age (and unfettered uncoolness) when you walk into a store like Granny's Goodies (that is, if you're over 25).
I don't understand how something from 1975 can be 'retro'--wasn't 1975 just like....a few years ago?
At any rate, Granny's Goodies has great stuff for the retro-loving kid who thinks (snicker) that the Brady Bunch wore COOL CLOTHES (heh), or great for you to walk down memory lane and realize just how old and out of touch you are. I find this to be a great source for halloween costumes. They have just about everything from pre-1990. Even really old things---I saw a dress I swear my mother wore to her prom in 1955....go figure!