As your targets are SME companies with 3 or more developers you can use LinkedIn premium search for finding the list of top executives of the companies and send them in-mails for partnership!
Another great source for you could be outsourcing websites where US development companies are represented and do testing for their clients periodically for various projects!

Please call for more detailed information of the best B2B sources for you! I have got also some availability to the actual lead generation job for you with a reasonable hourly rate. I have been working as a sales manager in this industry for over 4 years and have got large databases of B2B clients that you will be interested in!