The same way you would if you'd be marketing the product: either you need to spend time on free sources of traffic (ie. content marketing) or use paid source like pay per click to target exactly who you want to reach. Customer Validation forces you to think in terms of your path to market.
Advertising is an option. I've used it in past for customer development and discovery, although roughly 1 out of about 40 people who signed up for a sample were willing to be interviewed. Also...it's worth keeping in mind that you are establishing a relationship with each person (it's not just 1 customer development interview). They are often happy to continue engaging with you, regardless of how you established contact.