You sure you need someone's help with this right now? Yes, there are folks that kick ass at doing this, and I'm sure some of them are listed here right now. But as a startup, I'd be careful putting much money into this. It's not that hard to use a screencasting tool like ishowu hd for the mac and putting it up on viddler or youtube. We've done it, and it works. Do a little editing in iMovie or the Windows equivalents, and you have a screencast in a few hours of work.
Even 37signals does a lot of their own screencasts and they have a lot more money than most people do to be spending it on having someone else do them:
Click that "Watch how fast and easy it is to make a Backpack Page" link. I believe that's one of their employees talking, Matt Linderman.
I would start off doing a few of these on my own, and testing them on my site and blog before I start spending capital to have someone else do them.