Markets that accept horror/sci-fi short stories and pay semi-pro to pro rates are a constantly changing landscape (unfortunately). There’s little point in me listing currently open markets and then struggling to keep this page updated when so many other resources exist to do exactly that and do it far better than I could. You’re better off utilizing those resources. Some of the best are:
Duotrope – subscription-based but still an excellent site
The (Submission) Grinder – free and excellent
Dark Markets – online market guide for horror writers
Ralan – probably one of the oldest market resources around.
The Horror Tree – more than just a listing of horror markets.