SRDS is the Bible when it comes to finding publications by catagory, both US and some International. They also list Rate Cards for anyone planning an Ad budget, and are widely used in the Ad Industry. They came out with a new publication for Interactive media, which most of us should be listed in.
Basic listings are free, they charge the end user.
Here is the contact info. I have no affiliation with SRDS, but feel they are the best source of information, if building your own press release database. If you win an auction, what you bid is what you pay.