In a nutshell when he was building WPEngine he went to LinkedIn and found folks who were Wordpress consultants. He then sent them a follow email and said he's building a product for "folks like you and would love to talk to you about your pains, needs, etc" (customer development stuff) and offered to pay for their time. It worked well - he sent 40, 100% agreed to talk, actually talked to 38, and 0 asked for money. He suggests this worked so well because the offer to pay showed he was respectful of their time so they were happy to help. YMMV.