Start with https://PostCardMania.com which allows you to start with very good list, as they mail massively every day.

And a postcard sequence can quickly churn out (butter churn causing cream to rise) the "cream" of every list which will produce your highest profits.

These guys manage intersects + sorts of many other lists, so generally they have far "hotter" (more responsive) lists than other list brokers.

I've used Zoom in the past and found it was great as well.

From my experience running cost-effective email outreach campaigns, there are a few email address providers I'd recommend.

Hunter.io (https://hunter.io/) 50 free searches per month. Paid accounts start at $49/month for 1,000 searches ($0.049 per search).

Hunter provides you with emails based on the company domain. Even if it can't get you the right email, it'll provide you with the email pattern so you can make an educated guess. I've seen 70% success with Hunter.io which is pretty good for a low-cost solution.

RocketReach (https://rocketreach.co/) Paid accounts start at $7/month for 125 searches.

RocketReach is also a good solution that has given roughly 75% accuracy with data as well.

Snov.io (https://.snov.io) 50 free searches per month. Paid accounts start at $29 for 1,000 single searches or 500 bulk searches ($0.029 per search).

Snov.io is what I used to use. It basically scrapes email addresses and other data points off LinkedIn. I've seen roughly 65% accuracy on the data. The benefit is that it also scrapes phone numbers, website URLs, etc.

Low-cost labor on Upwork. You can hire low-cost workers on Upwork, which is a freelancer marketplace, to find and scrape high-quality emails. It's a very cost-effective solution and you can essentially hire a team of people to scrape emails for you in a time-efficient manner.

Finally, Ahrefs did an article where they tested different low-cost solutions that should help guide you: https://ahrefs.com/blog/find-email-address/