There isn't a single B2B category you could be trying to sell to that doesn't have at least a handful of active Twitter users, bloggers and enthusiastic early adopters who would be thrilled to give your product a try.
A simple place to start is Twitter. Use key industry search terms and reach-out to the people who have the most followers (but who are not followed by as many people), who have - in your own opinion - interesting things to say & share, and who actively engage in @discussions.
There are some people who just tweet out links but are not engaged in conversation on Twitter.
Once you've found these people, start following them and wait a couple of days at least. The best case scenario is to build a relationship with them by replying to and /or retweeting their links first.
Whenever you're ready to ask, be complimentary, briefly describe your ask and include a link with more information. Don't do many asks all at the same time, as it might be perceived to be spamming.
There are additional resources to cover but I've found Twitter to be an incredibly effective place to recruit users for a broad variety of applications. Happy to talk to you about this further, if you'd like.