A sales intelligence and lead generation platform that allows you to buy highly targeted email lists with a focus on real-time verification and good deliverability.
Lusha: A B2B data provider that helps businesses find accurate email addresses and phone numbers of professionals, often used for sales and recruiting.

Lead411: Provides updated contact information and company news, helping businesses identify prospects who are ready to buy.
UpLead: Offers real-time verified B2B contacts based on intent data, guaranteeing high accuracy.
Hunter.io: Helps you find email addresses country wise email marketing list associated with any website and verifies their authenticity.
Apollo.io: Offers tools for finding business leads, sending emails, and managing contacts, with features like email open tracking.

Seamless.ai: Uses AI to find emails and phone numbers in real-time, ensuring up-to-date contact lists.

These tools focus specifically on helping you grow your email list by converting website visitors into subscribers through various forms and pop-ups. They often integrate with your chosen ESP.
OptinMonster: Highly regarded for its powerful lead capture features, offering full control over where, when, and how pop-ups appear, with advanced behavioral targeting.