Try online resources. There are many online resources you could use to find Voice Over talent. I would recommend trying somewhere such as http://www.voices.com These sites offer sample recordings of a person's talent, which you can browse for free. Once you find someone you like you contact them. I believe the site takes a portion of the cost of the job off the top.
I've recorded many videos using professional voice-over as well as internal folks.
The main feedback I have received so far is that people have a tendency to trust more the voices that sound real.
This has basically to do with the fact that professional voice-over recordings are obviously better produced, with little paused and no hiccups... and therefore sound less credible!
Home made recordings (with good sound quality off course) give the sense that you have a real person talking - many times, using someone from the company makes it even more real.
The only watch out is that you need someone who is a good speaker, that doesn't have funny accents, and that is able to "talk with a smile". This last characteristic is fundamental, since your viewers are able to "see" the mood of who's talking and, if done well, will get energized by the voice.
Hope this helps
I would recommend putting a project on Vworker. When you put the project on this site you should ask for samples. You want to review samples of work and see if you think it is a good fit for your project. Also, you want to review the reviews that they received on this site. You want to make sure you select someone that has a lot of reviews and they has good reviews. When you look at the reviews, look at the bad reviews and see if you notice a pattern in the behavior. We have used vworker before and we have been very happy with them.