I notice that builtwith only has lists of websites spending a certain amount/using marketing automation etc. But how do I know those that are agencies? I want to reach out to every Drupal developer using BuiltWith. Any ideas?
BuiltWith is awesome, but not for this. It only shows you what sites are built with the tech, and often Magento / Drupal developers have websites for themselves that are on simpler platforms (for example, a Magento developer isn't selling products so they don't need an ecommerce platform like Magento for themselves).
Here's the list you need for Drupal:
https://www.drupal.org/drupal-services
Here's a list for Magento:
http://partners.magento.com/partner_locator/search.aspx
Hope that helps! Most platforms have developer / agency directories like this.
