You really don´t need to buy any list.
Everything you need is Google and have the Moz toolbar extension installed on Chrome and that´s pretty much it!
The easiest way to accomplish this is by leveraging the power of Google´s advanced search operators.
WRITE FOR US APPROACH
“[your keyword]” +”write for us”
keyword intitle:”write for us”
Keyword intitle:”write for me”
“[your keyword]” +”write for me”
“[your keyword]” +”become a contributor”
“[Your keyword]” + “guest post”
“[your keyword]” +”blogging guidelines”
Keyword + “guest post guidelines”
“[your keyword]” +”contribute”
“[your keyword]” + “guest column”
“[your keyword]” +”submit a guest post”
“[Your keyword]” + “accepting guest posts”
“[Your keyword]” + “Now Accepting Guest Posts”
“[your keyword]” +”contribute to this site”
Keyword intitle:”contribute to”
Keyword intitle:”submit” + inurl:blog
Keyword “submit a guest post”
Keyword inurl:/guest-post/
Keyword “guest post”
Keyword “guest post by”
Keyword “accepting guest posts”
Keyword “guest post guidelines”
Keyword “guest author”
Keyword “guest article”
Keyword “guest column”
Keyword “become a contributor”
inpostauthor:guest keyword
inpostauthor:”guest blog” keyword
inpostauthor:”guest post” keyword
FIND OPPORTUNITIES BASED ON OTHER GUEST POSTS
“[Your keyword]” + “guest post by”
“[Your keyword]” + “this is a guest contribution”
“[Your keyword]” + “contributing writer”
“[Your keyword]” + “guest author”
“[your keyword]” + the following guest post”
“[Your keyword]” + “guest blogger”
“[Your keyword]” + “guest column”
“[Your keyword]” + “guest article”
IN URL HACKS
“[Your keyword]”+ inurl:guest-posts
“[Your keyword]” + inurl:write-for-us
“[Your keyword]” + inurl:guest-post-guidelines
“[Your keyword]” + inurl

rofiles/blog/new
“[Your keyword]”+ inurl:category/guest
“[Your keyword]”+ inurl:contributors
GO WHERE YOUR COMPETITOR’S / HERO WRITES:
“Guest post” + “[influencer name]”
“Contributor” +“[influencer name]”
I think this will be enough to keep you busy for a few days until you build a pretty solid list of authoritative and relevant sites within your niche that are accepting guest posts.
After that, all you need to do is run your own outreach campaign.
I know this sounds easier than it looks but, there are plenty of resources from where you can learn the basics of the process and even download templates you can use on your email outreach campaigns.
Remember, don´t get discouraged if you sent out 100 emails and get only 1 response. All you need is one (1) high-quality and relevant link from an authority website in your niche to have a huge impact on your page ranking.
Good luck!!