Voice over jobs are posted by the dozens every day on just one site. So there are probably hundreds or thousands available daily. Many of those won't be available to the public, but the ones on the site I mentioned before definitely are. That site is Voices.Com. It's free to sign up for a profile, and as soon as you do you get to start putting up samples of you voice recordings.
That last part might scare some people, but don't let it put you off at all. If you have a computer, you won't need to spend more than about $49 to start making professional sounding voice over recordings. And that's only if you don't already have a half-way decent mic. $59 is about the minimum price for a USB mic (such as the Samson Q2U) that can yield good quality audio. Then if you don't have any voice recording software yet, simply download the free tracking and mixing program called Audacity. To figure out how to make recordings with Audacity, do an internet search for tutorials. Or you could check out the Home Brew Audio tutorials on Audacity here: Recording Tutorials.
Once you have some demo recordings you can upload them to your Voices.com profile. But the real potential is in auditioning for open jobs. Voices.com will send you notices when voice over jobs get posted, and all you have to do is go to the listing for the job, find out what kind of voice-over they want, quote your price and send the recording in for your audition.
Each job posting will have specific details, like what their budget range is (the minimum is $100), and what type of voice they need. The listing will also usually (I'd say 90 percent of the time) contain an attached script that will have parts of what the client is looking for. I highly recommend you record from that script for your audition.
This is called a "custom" audition and will be much more targeted to what the client needs, which will increase your chances of getting the job. One thing to be careful of at this stage is protecting your recording.
If a client is looking for just one paragraph for the entire job, and they provide that paragraph as the audition, anyone who reads the entire thing will have given the client what they need already. Less scrupulous folks can (and have) simply use the audition to fill their job without having to pay you. Preventing this is pretty easy. Either don't read 100% of the script, or insert a "watermark" to make it impractical for the the audition to be used as a final product.
Here is where knowing the reality of a situation can make the difference between success and failure. Before doing any of this, you should know that as with any acting job, voice over jobs are highly competitive. I would venture a guess that you will get a job about every 100 auditions. This sounds bad but really it isn't. If you know it going in, you won't be ready to throw in the towel when your 50th audition still has not yielded one job. Do at least 100 before even starting to get discouraged. The people who succeed in this business are the ones who are persistent and focused.
There are lots of tips on the web about how to make the best recordings for voice overs, how to increase you odds of getting voice over jobs, etc. One such source of advice is the Home Brew Audio website. However you decide to proceed, good luck in your new voice-over career!