I like the old 'B' science fiction movies, the sort that used to be considered drive-in movies in the late 50s/early 60s. These are hard to find anymore, either on tape or on television. I've recently found two good sources for them, however:
Tapes of Terror leans mainly toward horror and slasher movies from the 70s and 80s. What few older films they have are true gems though. I got my copy of "Wizard of Mars" from T of T, a favourite movie that otherwise seems to have vanished from the face of the earth. They've also got classics such as "Frankenstein vs. the Space Monster" and "They Saved Hitler's Brain", neither of which you're likely to rent from your neighborhood Lackluster Video. On the good side they list over 2000 titles in their catalogue. On the bad side, there's no real descriptions for most titles, so you've got to know what you're looking for. They've got more stuff than is shown on their website, BTW. If you're looking for something in particular write to Morticia and ask her.
Sinister Cinema carries a wide variety of films listed by genre, with a good capsule description for each title. While they don't have as many titles as Tapes of Terror, just about every title they do have is something that sounds as though I'd enjoy watching it. They've got old serials, westerns, jungle movies, detective movies, etcetera, and almost all of them made before 1960.
If anyone else has a favourite source for movies of this type, I'd be pleased to hear of it.