LinkedIn Sales Navigator, NerdyData, and Apollo are cost-effective options.
Inbound is the best way to get to them. Depending on your use case, cold outbound outreach may make sense. In that case, Navigator with LeadIQ (Alchemist Alumni) is the best and most cost-effective combo. LeadIQ directly integrates with Sales Navigator as well as outreach (and Salesloft). You can do the search in Navigator and directly import/update a contact in Outreach. There is a 50% Alchemist Discount as well.