Hi Raquel, we had an event here on the forums a while back with Diana Ennen about press releases,
here's an archive of that chat, she gives lots of good info that might be useful to you, including a few sites she prefers for submission of releases.
I personally use PRWeb for paid submission - they have great
SEO features and releases always make it into the Google News stream, which is ideal.
Also, check the
Useful Resources section of the VAF Directory for a list of release submission sites both paid and free.