Look for the most important competitions for scripts in general. They sometimes have a category for shorts.
shameless plug: www.writersandfilmmakers.com
The short film competition deadline is Feb 29th, 2016. Still working on the feature....it's going sloooow
I like the writersandfilmmakers-concept a lot - movie makers evaluate the scripts and the other way around. Winners move on and the script is shot. Brilliant.
Thank you firm1! I didn't check them yet but I think these 10 are not free!