I'd contact the leaders of professional or interest groups related to your domain and ask if they can recommend some members who might be interested. If you contact that person in a respectful and professional way, they'll feel slightly more obliged not to dismiss you immediately, because they're acting in a semi-official capacity. And if you get through to them, they can pass you on to members who will take you more seriously because you come with the imprimatur of someone they know.
[Before, during, and after the session you treat them well, don't waste their time, thank them, etc., which hopefully goes without saying]
Then when you're done with each interview, you ask the participant "hey, this was so great, can you recommend anybody else I should talk to?"
To that last sentence, I often used to add "... I'm looking for the most outspoken, opinionated expert you know, the person with no filter, who just tells it like it is". Chances are they know a person like that, and that person immediately leapt to their mind. That's who you really want to talk to anyway, the person who will tell you exactly what is wrong with everything out there.
In terms of asking people about pain points, the best approach I've used is to get them to list their pain points, then ask them to stack rank that list. If applicable, ask them how much they'd pay for a product that solved each of those problems, or just the top n. You want to know whether they're just brainstorming minor quibbles, or if there is actually an opportunity there.