Putting an ad in the classifieds is probably not going to yield many results. You need to go where the artists are. Online, one such place is deviantArt. Thousands of pieces of art are uploaded to the site daily. You can browse people's portfolios and find the style of art that you want. Then place an ad in the forums. A ton of starving artists will likely respond within minutes. This is the phase I found myself in earlier this week. As I weed through the candidates, there are a few things I would have done differently up front.
First, as I said, having found a number of favorites first is important, so you and the artist share a frame of reference of what you expect. To do this you need a free account. Fortunately, I set one up a few years ago. I imagine some people would be turned off by a "newbie" coming on looking to hire.
Another key is to have what you are looking for as detailed as possible up front. Many artists are leery of giving a set price since the amount of work can vary considerably depending on the details. It's frustrating to get a lot of responses and not have prices, so there's no way to really compare or budget. Also, one thing I initially overlooked was specifying a commercial license. Some people charge differently than for personal pieces.
To deal with all the hassle I was encountering, I ended up making a page on this website. It lists all the commission details, including mock ups of the expected artwork, and it has a form for the applicant to fill out. They have to specify what job they're interested in and, importantly, list a price range. All the responses are nicely arrayed in an online database, so I don't have to do a lot of the record keeping. I wish I had done this right away.
All this being said, I haven't completed the whole process yet. I'm sure there will be some more snags along the way, but maybe my experience so far might help someone else make their first steps toward getting that artwork that's just right. Or if you have other advise, leave a comment.