Where can I find a dataset of direct mail response rates?

What industry do you need it for? PostcardMania might be able to tell you as we track and analyze our customers' campaigns. Check out: http://www.postcardmania.com But response rate is not really what you should use to measure the success of a direct mail postcard campaign (or any marketing push for that matter). What you need to look at is return on investment. This means how much money in terms of leads and sales the campaign produced in comparison to how much it cost. You see, it is possible for a campaign to actually produce a lot in terms of revenue for you, but not actually produce that many responses. An example of this is if you have a very high-price item you are selling. You may only get a handful of responses, but if two or three people buy you could pay for the campaign twice over. That would be a marketing strategy you should stick with, even though the response rate doesn’t appear to be that great. Another example would be someone who sells a lower price item that is a recurring purchase. In this case, you need to factor in how often an average customer buys from you in order to know just how valuable that lead really is. Maybe you don’t turn a profit on the first sale, but by the fifth you do. PostcardMania works with this kind of model.

Try looking at "who's mailing what" or the trade journal for direct marketing.