FameBit - Easiest Way to Find, Hire, and Work with YouTube Influencers
Really Good Emails - A large collection of good product email design.
inzpire.me - An end-to-end solution for influencer marketing
Email Love - Email design inspiration, templates and discovery
Insense & Paid Media - Find creators for short-term and long-term partnerships
Stamplia - High converting and beautifully designed email templates