I agree with the OP. 

I have only done 2 flicks of note and they both have lead actresses.  That is not an accident.  I want to make it a point to have a female lead going forward (I'm a guy btw) but it is so much harder to produce when you are casting women.  Why?  If you have an all male cast then finding actors is guaranteed (time is the only variable) meaning you can do scripts and animate non-dialogue scenes without a worry.  If you hope to cast one or more women then you need to script first, get committment 2nd, the get dialogue and then, only then, are you brave enough to animate because if you fail to find an actress then all the scenes with a female minifig need to be redone - probably your entire film. 

I think the only fix is for communities like ours to pledge to star girls in speaking roles for every film released in the next 6 months or so.  Create a demand so great the voice acting communty will learn that they are needed and wanted and free practice is available.

Dunno - hard to turn the tide on my own.

I recommend two sources:
1) http://voiceactingalliance.com
2) Going on YouTube, use keywords such as "My Sims 3 audition", and directly contacting female actresses who have the best videos.  Good for teen voices.  1 in 10 is excellent.

Here's some more detailed information on finding actresses at these sources from my Brickfilming Guide:

Thanks for the help! I contacted someone on the first site to be in my film. Waiting for a response.

I don't know how helpful this is, but I have found that on sites like Voice Acting Alliance if you have a good pitch and some strong samples of previous work you've made, you can get a lot of auditions from talented people. I had a really positive experience trying this, they were excited at the opportunity to do something other than the bulk of what that site offers, anime fandubs and flash animations.