The ideas below are solely for researching already established actresses. There are many more tools available when doing casting calls and searching for lesser known actresses. This list also assumes you are casting the leads yourself and not using a casting director. Many indie producers cast the leads in their films themselves. I enjoy casting so I usually cast the leads myself and then hire a casting director to help handle the execution of the deals and to cast the smaller roles.
IMDb Pro's Starmeter
The first step in my research on actors is the IMDb Pro Starmeter. I scroll through it looking for actors in the right age range. I look at everyone I can find in the Starmeter with a ranking of at least 4,000 or above. I keep within that range because it literally takes hours to scroll through this list and most of the more popular actors are in the range of 4,000 and above. I feel confident that I will come across more names in my other areas of research that fall over the 4,000 mark. I then use the IMDb entry to go over the actor's resume and images, get age and height and contact info, and personal details that may help me and the director decide if this actor is the right choice for the part we have available.
The ole Internet search on actors is a must. I will search by specific actor name or do a general search on up-and-coming actors or award-winning actors or Latino actors, etc. The search terms can be endless. For example, my current search is for an up-and-coming actress so I found articles like this one from AskMen.com or this one from Gunaxin. I love themovie-fanatic.com for articles on highly acclaimed rising actors, but their site is down right now (very sad). Over time, you will find actresses who are consistently found on every up-and-comer list but you will also find gems you hadn't thought of or perhaps never came across.
Entertainment or Celebrity or Trade Magazines
Magazines such as Hollywood Reporter, Variety, Entertainment Weekly, People or Us Weekly are other great resources for articles on actors. And it gives you an excuse to get your fill of gossip too! Just admit that celebrity gossip is a guilty pleasure.
Other Independent Films
I also check out indie films that premiered at festivals or made a splash from the last five years or so to see who starred in them. This is a great way to find talent who are already predisposed to appearing in smaller, independent films. Once I get a list of films, I go to IMDb and search on who starred in them.
Don't forget TV actors. Often they have a film resume as well and they can bring a certain amount of domestic box office value with them if they are a series regular and get a lot of press from their TV work.
Many actors, even up and comers, have fan sites with a gallery of images and even trailers or clips from interviews and films.
Most actors have a few videos featuring their work on YouTube. I love to key in actors' names in YouTube and watch reels, scenes, and interviews so I can get a sense of a personality of an actor above and beyond their character work.
Go to the Video Store or Troll NetFlix
Sometimes I will go to the video store or troll NetFlix and scan new titles and genres that are similar to the film I am casting. DVD jackets usually have pictures of the cast and may present an actor who may be right.
And last but not least...
Agents and Managers
I also put the word out to agents and managers about my search. I have to be honest here. Agents and managers are very nice but it is hard to get them overly excited about a small, indie film. Definitely put the word out to them but don't get discouraged if they don't offer up any suggestions for you. Most of the work I do with them is after I have already figured out who I want to go to and then I try to make the project as appealing as possible so the agents and managers will be willing to promote the project to their client.
An effective way to find new ideas is to note the agents and managers who you find are consistently representing cast right for the part. Look at the client list of these agents and managers on IMDb and you may come across a new name there as well!