It’s amazing the amount of classic, and not so classic SF is online, for free. This Popular Mechanics article has a start.
There are more, such as the entire H.Beam Piper collection.
Even better, the entire run of Galaxy Magazine and all the classic SF is also online.
As is Heavy Metal.
Of course for modern SF, Baen has been offering ebooks for free in the free library right from the start.
So, for anybody with a computeraccess to most of the SF universe is available, a good part for free. Go out and discover.