I don't think production company emails are any harder than for agents certainly -- and maybe about the same as for managers.
Really big production companies stay off the radar. But there are a bunch of places making stuff under 20M that you can look up via IMDb and at least find the email of one or two producers, or the CEs.
What makes it hard is that there are an awful lot of them and the smaller ones do specialize in genres. The best method I've found is to grind it out and go through a bunch of movies similar in feel, genre, and budget to your script. Then click on every producer or X-producer on the list and collect emails.
Every script read you get this way, even if it doesn't result in a sale, turns into someone who's more likely to read your stuff in the future.
Steven Palmer Peterson