Ask HN: PMs/UX researchers – wisdom on hiring user study participants (B2B)?

Product Managers and UX researchers of HN, how do you recruit users for running studies for new products in B2B markets?

When the usual consumer market techniques (Craigslist/ Starbucks) don't apply, what have you seen work well? Are there any resources you can point me to? Super interested in learning from your experiences, anecdotal or otherwise.