Vault would be one source, Glassdoor would be similar. Linkedin can be your best route in this. What you are looking to do is make strong applications to a set number of mid sized firms, not send your resume everywhere. So identify which firms you know of which suit your target, then identify who their competitors are.
Can you give us some examples of the firms you are looking at as being medium-large? I'm sure we can put some more suggestions in the box which are alike to the ones you have already identified.