On one hand we have designers readily available, people we can hire to create something with direction or concepts already available.
On the other hand, we have sites like 99 Designs and Crowdspring to help one find the concept or direction that is right for them by getting samples from professionals.
I'm looking for the web service that falls in between. When doing illustrations or a series of designs, a company needs to find an illustrator who has the style they want though that concept may not be clear. One needs a forum through which possibilities can be evaluated for a host of designs rather than a single logo or page template.
What's the best way to find and get bids from a variety of illustrators/designers given only vague direction about the illustrations you need?
It sounds like you're looking for a creative designer as opposed to a graphic designer. I may be using those terms loosely, but I would define the former as someone who can start with a rough concept and bring it to life in their own style, or fashioned after some examples of work that you like. While the latter is more handy at taking pretty clear direction and digitizing it.
If I'm accurate in what you're looking for, I think 99designs may be the best bet for that, because many designers will present their own take on your objectives and you end up with some pretty cool results. If you have a clear idea of what you want, you can be very specific - or give some general guidance and sit back and see what they come up with. In either case, I've always been happy with the results.
If you're looking for a long-term designer, I would suggest finding someone who's work you love, or who has demonstrated the ability to take abstract concepts and turn them into great visual results. I don't know of a forum for finding those long-term types, but I have given repeat projects to designers I've had good work from on 99Designs, eLance, etc.
If you're looking for bids and the ability to see results before paying, 99d is the most trafficked that I know of. Good luck!
You might try a portfolio site like carbonmade - http://www.carbonmade.com/portfolios/ - they are the biggest I know of. You can search by skill or areas of expertise/industry and search for work. It might be a little more time consuming to search through the site, but overall I think you'll find higher quality designers, and art directors who are willing to take your project from conception to reality.
You may also want to try crowdspring.com (which is like 99designs). I haven't used 99designs (though I plan to in the near future) but crowdspring does bring in a strong yield of designs. The catch is to ensure that your award is high enough to be worth a designer/illustrators time to attempt to "win".
One thing to know about these design competition sites is that you should not necessarily expect the creative to be spot on with their submission the first time around - they will be using hints from your comments on other submissions as they are submitted.