Hey there - I do regularly work with email collection for specific niches, specific to b2b - often IT and software adjacent. Let me know what you're looking for and maybe I can help
Depends on what a niche means, here. How specific are you looking to slice which sorts of industries or psychographics? How many prospects how fast? Etc.
There should be some providers out there on this site - I'd be really surprised if nobody was selling emails. I think I saw some 7 million list here a week ago for ridiculously cheap.
Sorry - I don't know what's really /cheapest/ - since I don't use those free tools anymore. There are some browser plugins you can use to get a few targeted emails I think for free Lusha? Snov.io? I know you can just find them by googling how to find email addresses.
I know there are new and inexpensive things like phantombuster/hexomatic - they're still a bit clunky and nascent but you can usually just turn them on and let them run, which is nice:
https://support.phantombuster.com/hc/en-001/articles/360011556819-Get-emails-with-PhantomBuster
If you have sales nav, scrapehype has been pretty good at finding contacts for me, but after I scrub them through clearout I usually end up with 30-40% of the total. A lot of firms like apollo.io - but I find the user interface messy.
Serious - you can find a hundred other tools and services and plugins out there to scrape emails just by regular old googling. There are lots of ways to find ém. If you're just looking to test out a new product and don't know how to find the prospects it might be worth finding someone who can get you a nice list quickly. Upwork or something if you can't find a provider here?
Good luck