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.

I believe this link will help you in a much better way to solve your problem. Have a look at it: https://trends.builtwith.com/websitelist/Agency
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath