There are no best sources. It all depends on your target market and Ideal Customer Profile.
Based on my experience of selling B2B custom data for the last 2 years I am sharing a few things that you should keep in mind while finalising your data vendor are:
If you would like you can request a free sample data from EasyLeadz and see if it fits it your requirement.
Full disclosure: I am the founder of EasyLeadz.
P.S: If you like this post then your upvote will motivate me to write more.