It might not cost as much as you think to have a lawyer generate an NDA for you. We've had our lawyer generate 10 or so general contracts that we can modify to use in most situations, which I would recommend to any business owner.
Most lawyers have these already, and although they'll certainly charge you for the domain knowledge, you shouldn't have to pay hourly. You should be able to get a good, generic NDA for <= $500. NDAs are really simple, and usually pretty short.
I've never tried docstoc et. all, because part of the value for me is having a domain expert say "yes, this will cover your specific business needs."
One other thing: I've seen a number of companies providing their NDA on the contact / quote request page. I've been considering the same, as it might be a good way to head these uncomfortable conversations off at the pass.
Edit: this is a great place to barter as well. What you want from the lawyer is their expertise, not their time, and lawyers know the ROI a good website can bring them. I'm sure you know some lawyers, ask one of them if they'd be willing to barter a website in return for a set of documents. If you happen to be an SEO (I know, dirty word around here), offer that instead - one of my clients is an attorney, and the #2 Google ranking we helped him achieve yields thousands of dollars each month.
