What's the best source or tool out there to find Fortune1000 email addresses?

I've used this a bit - seems legit!

Especially for Fortune 1000, it's easy to guess the general format that an organization uses. You may use Hunter.io for free and see that Company X generally uses @.com I use Vocus.io's to verify if my guessed email address is correct, preventing bounced emails

Hi Nikhil, there are many tools or source you may find for fetching email address also, at the same time you may find a lot of staled and old data because of which you may face huge bounce of rate but AeroLeads is one best email finding tool which can source email for any of your requirement and provide you more than 95% accurate data. They also have their own services team who can build a fresh list for you within 24 hours.

Just view their LinkedIn profile. This one works fairly often.