You can try websites like Project Gutenberg. It has a large collection of classic science fiction works available for free in e - book format. Another option is Amazon Kindle Store. It offers a wide range of science fiction e - books, both free and paid. You can also check out Smashwords, which has works from independent authors.
Well, there are several places. Goodreads is a great start. Although it's mainly a review site, it often has links to where you can get the books. Some libraries also have e - lending services for online books. For example, OverDrive is used by many libraries. Just search for science fiction in their e - book catalogs. And don't forget Google Play Books. They have a decent selection of science fiction titles too.
There are multiple sources for science fiction online books. One is Tor.com. They not only publish great science fiction stories on their website but also sometimes offer e - book versions of their works. Additionally, Wattpad has a lot of user - generated science fiction content. Some of it is really good. You can also explore the Baen Free Library. It specializes in science fiction and fantasy and offers a good number of free e - books.