Edit1: Caitlin's Graphical World is a big mixture of stuff, with modern/Sci-Fi/Post-apocalyptic stuff included. Then Joy's Futuristic Tiles - Spaceship Interior for a nice collection of edits. Lunarea & Celianna's Steampunk tiles can also be used (though they're in parallax format, so some work needed)
Also Check out IcyDragon's Modern/Futeristic tiles specifically, the topic also has a handy one-stop download link for the whole thing. Some stuff from Enterbrain/Tsukuru's Blog GrandmaDeb's Modern Exterior tiles (as Sci-fi often uses things that can appear modern as well) She also has quite a few links within both posts to stuff that is certainly Sci-Fi.
First check out both Granny's Lists - Modern and Futuristic Resources and Word of Warning, some things might have watermarks from photobucket or whatever, so you might need to use something like Gimp or Photoshop to try and fix that. Click to expand.This is certainly one of the more difficult things to obtain, but thankfully we have a lot of people who have helped make this easier.
