You can try websites like Project Gutenberg. It has a large collection of free e-books, including many classic science fiction works. Another option is Librivox if you prefer audiobook versions of science fiction e-books, as they are also free.
There are also some free e - book aggregator sites. For example, Feedbooks has a section dedicated to free public - domain and original - license science fiction e - books. You just need to search through their catalogue to find the ones that interest you.