Facebook SPI Group: This is a great group and an active group of over 6,000 fans of SPI games who are very knowledgeable and helpful when you are looking for parts or history or someone to play. Just a super bunch of people.
Board Game Geek (BGG) SPI games: BGG is another great site. BGG has a list (with multiple pages) of every one of SPI’s games. You can buy, sell, and trade on BGG, and it can be a huge asset if you’re looking for parts for a game.
SPIGAMES.NET: There is no bigger fan or accomplished historian of SPI than Russ Gifford, and his site shows it. He has a tremendous collection of resources on this site — everything from copies of the rules, to numerous game write-ups, and histories of various aspects of SPI production (which games came in which box, etc.). Russ also has a tool for getting people linked up to play SPI games (in person or online). A site you must bookmark if you play SPI games.
SIMPUBS.ORG: This site is managed by John Edwards, another loving historian of SPI games. Organized differently than Russ’ site, John’s site nicely and consistently groups each game into its own page where you can find all the resources and artwork for that game in one spot. He often has high-resolution images you can download and print.