Threefold for the ones I haven’t seen: 

1) Things I’ve heard of over the years in passing, like seeing a bit on a movie channel or something

2) Tumblr blogs like trash-fuckyou and moviesludge for example, have a seen a ton more than I have pick some of those. They’re awesome for the weird, z-grade and cult classics. 

3) Plain research, wikipedia has a list of 80s science fiction movies, imdb recommendations, amazon recommendations. I have things on my amazon wish list that are 10 years old that I’m finally watching.

That’s not to say I haven’t spent my share of late nights watching bad movies in the past (when I had cable). I’ve watched terrible shit, and obscure shit.