What sort of price are you looking to pay? I've written a lot of secure software in the past, and I may well be able to recommend a couple of firms who have done a similar job for me but I'm afraid they don't come terribly cheap.
Off the top of my head (which is likely to be way off), I was expecting something between $1000 - $3000
What is it you're actually wanting audited, and what is at stake if it turns out to be broken?
If you're reasonably confident that you've got a decent security model, and you've coded it defensively you're probably OK. I wouldn't stress about it too much at this point.
Thanks a lot for the advice. I really appreciate it.