Friends and families: Use your personal connections through email and social media channels and ask for their time (no more than 30 minutes) to conduct contextual inquiries and get validation on your prototypes. Most people won’t say no. Do thank them by giving them a small gift or a gift card.
Find paid end users: This does seem odd but it works. I know of a few start-ups that have used this method effectively. Use Craigslist and other means to recruit people that match your end user role and pay them to participate in feedback sessions. It is worth it.
Guerrilla style: Go to a convention or a conference where you could find enough end users that fit your profile. Camp out at the convention with swag and run guerrilla style recruiting to validate and prototype. Iterate quickly, preferably in front of them, and validate again.
(Cross-posted @ cloud computing)