Confused - Finance a Down Payment through a Private Lender?

[Zach Norris]

Hey! I'm very excited to be starting my real estate career and am looking at buying my first rental property as soon as I can. I've been reading a ton of the information on BiggerPockets but still find myself without a solid understanding on the specifics of using money from a private lender / investor to finance a down payment for a rental property. 

Somewhere, I read / heard that it wasn't a bad idea for my first purchase to talk to family members about maybe taking a loan out of their retirement account to lend me, which I would pay back with interest. Is this a legitimate route I can take in order to acquire funding for my first down payment? If so, how does that actually work? 

Thanks for any / all responses and advice!!!