Even if they aren't paying for the work done, you're still asking people to send you models, which
aren't free. I'm sure that more than a few people will still be uncomfortable with the idea, since they'd be going in blind. I'd start looking for work locally, if possible, since people who have personally met you and can more easily track you down may have fewer reservations about sending models home with you.
As for the portfolio, what's to stop you from using your own models? Yes, you need to establish both the quality of your work
and the reliability of your service, but there's nothing to say you can't expand on either of those points, individually. The more you show off your skills, the more likely people will be to take the plunge (or at list dip in a toe

), even if you aren't that well established, yet.
That's my feeling, at least. I'm sure that actual commission painters (there are more than a few, here) will have more to say on the matter, and with more weight.